Refactor timeline event list

What does this MR do and why?

Describe in detail what your merge request does and why.

In order to allow that addition of edit functionality in timeline events as in #344060 (closed), the timeline event list must be refactored so that app/assets/javascripts/issues/show/components/incidents/timeline_events_form.vue can be used within the list.

No visual changes, only refactoring

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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app/assets/javascripts/issues/show/components/incidents/timeline_events_item.vue

 <script>
-import { GlDropdown, GlDropdownItem, GlIcon, GlSafeHtmlDirective, GlSprintf } from '@gitlab/ui';
+import {
+  GlButton,
+  GlDropdown,
+  GlDropdownItem,
+  GlIcon,
+  GlSafeHtmlDirective,
+  GlSprintf,
+} from '@gitlab/ui';
 import { formatDate } from '~/lib/utils/datetime_utility';
 import { __ } from '~/locale';
 import { getEventIcon } from './utils';
@@ -12,6 +19,7 @@ export default {
     timeUTC: __('%{time} UTC'),
   },
   components: {
+    GlButton,
     GlDropdown,
     GlDropdownItem,
     GlIcon,