Disable "Remove source branch" in MR Widget for users who can't remove, and re-add checkbox to MR form

What does this MR do?

This MR does three things:

1. Re-adds the "Remove source branch" checkbox to the Create/Edit MR page
2. Ensures the "Remove source branch" checkbox in the MR widget is disabled for users who do not have rights to delete the source branch
3. Makes the "Remove source branch" checkbox in the MR Widget dependent on initial MR form settings, rather than defaulting to removing.

Closes #32447 (closed)

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/32907

app/views/shared/issuable/form/_merge_params.html.haml

@@ -5,3 +5,12 @@
 
 -# This check is duplicated below, to avoid conflicts with EE.
 - return unless issuable.can_remove_source_branch?(current_user)
+
+.form-group
+  .col-sm-10.col-sm-offset-2
+    - if issuable.can_remove_source_branch?(current_user)
+      .checkbox
+        = label_tag 'merge_request[force_remove_source_branch]' do
+          = hidden_field_tag 'merge_request[force_remove_source_branch]', '0', id: nil
+          = check_box_tag 'merge_request[force_remove_source_branch]', '1', issuable.force_remove_source_branch?
+          Remove source branch when merge request is accepted.