Improve delete merged branches modal

What does this MR do and why?

Improves deleting all merged branches experience by providing more clear messages and requesting user input regarding the amount of branches beeing deleted.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to Repository > Branches
2. Click on 'Delete merged branches' button. (If you don't see one you need to create some merged branches).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #20384 (closed)

app/assets/javascripts/branches/components/delete_merged_branches.vue

+<script>
+import { GlButton, GlFormInput, GlModal, GlSprintf, GlTooltipDirective } from '@gitlab/ui';
+import csrf from '~/lib/utils/csrf';
+import { sprintf, s__, __ } from '~/locale';
+
+export const i18n = {
+  deleteButtonText: s__('Branches|Delete merged branches'),
+  buttonTooltipText: s__("Branches|Delete all branches that are merged into '%{defaultBranch}'"),
+  modalTitle: s__('Branches|Delete all merged branches?'),
+  modalTitleProtectedBranch: s__('Branches|Delete protected branch. Are you ABSOLUTELY SURE?'),
+  modalMessage: s__(
+    'Branches|You are about to %{strongStart}delete all branches%{strongEnd} that were merged into %{codeStart}%{defaultBranch}%{codeEnd}.',
+  ),
+  notVisibleBranchesWarning: s__(
+    'Branches|This may include merged branches that are not visible on the current screen.',
+  ),
+  protectedBranchWarning: s__(
+    "Branches|A branch won't be deleted if it is protected or associated with an open merge request.",
+  ),
+  permanentEffectWarning: s__(
+    'Branches|This bulk action is %{strongStart}permanent and cannot be undone or recovered%{strongEnd}.',
+  ),
+  confirmationMessage: s__(
+    'Branches|Plese type the following to confirm: %{codeStart}delete%{codeEnd}.',
+  ),
+  cancelButtonText: __('Cancel'),
+  showMore: s__('Branches|Show more'),
+  showLess: s__('Branches|Show Less'),
+};
+
+export default {
+  csrf,
+  components: {
+    GlModal,
+    GlButton,
+    GlFormInput,
+    GlSprintf,
+  },
+  directives: {
+    GlTooltip: GlTooltipDirective,
+  },
+  props: {
+    formPath: {
+      type: String,
+      required: true,
+    },
+    defaultBranch: {
+      type: String,
+      required: true,
+    },
+  },
+  data() {
+    return {
+      areAllBranchesVisible: false,
+      enteredText: '',
+    };
+  },
+  computed: {
+    buttonTooltipText() {
+      return sprintf(this.$options.i18n.buttonTooltipText, { defaultBranch: this.defaultBranch });
+    },
+    modalMessage() {
+      return sprintf(this.$options.i18n.modalMessage, {
+        defaultBranch: this.defaultBranch,
+      });
+    },
+    isDeletingConfirmed() {
+      return this.enteredText.trim().toLowerCase() === 'delete';
+    },
+    isDeleteButtonDisabled() {
+      return !this.isDeletingConfirmed;
+    },
+  },
+  methods: {
+    openModal() {
+      this.$refs.modal.show();
+    },
+    submitForm() {
+      this.$refs.form.submit();
+    },
+    closeModal() {
+      this.$refs.modal.hide();
+    },
+  },
+  i18n,
+};
+</script>
+
+<template>
+  <div>
+    <gl-button
+      v-gl-tooltip="buttonTooltipText"
+      class="gl-mr-3"
+      data-qa-selector="delete_merged_branches_button"
+      category="secondary"
+      variant="danger"
+      @click="openModal"
+      >{{ $options.i18n.deleteButtonText }}
+    </gl-button>
+    <gl-modal
+      ref="modal"
+      size="sm"
+      modal-id="delete-merged-branches"
+      :title="$options.i18n.modalTitle"
+    >
+      <form ref="form" :action="formPath" method="post">
+        <p>
+          <gl-sprintf :message="modalMessage">
+            <template #strong="{ content }">
+              <strong>{{ content }}</strong>
+            </template>
+            <template #code="{ content }">
+              <code>{{ content }}</code>
+            </template>
+          </gl-sprintf>
+        </p>
+        <p>
+          {{ $options.i18n.notVisibleBranchesWarning }}
+        </p>
+        <p>
+          {{ $options.i18n.protectedBranchWarning }}
+        </p>
+        <p>
+          <gl-sprintf :message="$options.i18n.permanentEffectWarning">
+            <template #strong="{ content }">
+              <strong>{{ content }}</strong>
+            </template>
+          </gl-sprintf>
+        </p>
+        <p>
+          <gl-sprintf :message="$options.i18n.confirmationMessage">
+            <template #code="{ content }">
+              <code>{{ content }}</code>
+            </template>
+          </gl-sprintf>
+          <gl-form-input
+            v-model="enteredText"
+            data-qa-selector="delete_merged_branches_input"
+            type="text"
+            size="sm"
+            class="gl-mt-2"
+            aria-labelledby="input-label"
+            autocomplete="off"
+          />
+        </p>
+
+        <input ref="method" type="hidden" name="_method" value="delete" />
+        <input :value="$options.csrf.token" type="hidden" name="authenticity_token" />
+      </form>
+
+      <template #modal-footer>
+        <div
+          class="gl-display-flex gl-flex-direction-row gl-justify-content-end gl-flex-wrap gl-m-0 gl-mr-3"
+        >
+          <gl-button data-testid="delete-merged-branches-cancel-button" @click="closeModal">
+            {{ $options.i18n.cancelButtonText }}
+          </gl-button>
+          <gl-button
+            ref="deleteMergedBrancesButton"
+            :disabled="isDeleteButtonDisabled"
+            variant="danger"
+            data-qa-selector="delete_merged_branches_confirmation_button"
+            data-testid="delete-merged-branches-confirmation-button"
+            @click="submitForm"
+            >{{ $options.i18n.deleteButtonText }}</gl-button
+          >
+        </div>
+      </template>
+    </gl-modal>
+  </div>
+</template>