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

spec/features/merge_requests/created_from_fork_spec.rb

@@ -63,6 +63,21 @@
     end
   end
 
+  context 'user can merge into source project but cannot push to fork' do
+    given(:user2) { create(:user) }
+
+    background do
+      project.team << [user2, :master]
+      logout
+      login_as user2
+      visit_merge_request(merge_request)
+    end
+
+    scenario 'user cannot remove source branch', js: true do
+      expect(page).to have_field('remove-source-branch-input', disabled: true)
+    end
+  end
+
   def visit_merge_request(mr)
     visit namespace_project_merge_request_path(project.namespace,
                                                project, mr)

[Oswaldo Ferreira]

@brycepj Do you mind moving this to spec/features/merge_requests/widget_spec.rb?

That file actually tests forks of forks :) I've mentioned the setup as a guidance actually.

[Bryce Johnson]

Discussion moved here and resolved 👍

[Oswaldo Ferreira]

Can we have a new scenario like user can merge the merge request, by checking we can see a non-disabled Merge button?

[Bryce Johnson]

Discussion moved here and resolved 👍