Show the suggestion onboarding popover in Rapid Diffs

What

On legacy diffs, a first-time user who opens a comment form on a diff line sees a one-time onboarding popover pointing at code suggestions, with a "Got it" button that dismisses it permanently for that user. Rapid Diffs never showed this popover and did not offer code suggestions on new-line comment forms at all. This restores that popover and re-enables spec/features/merge_request/user_suggests_changes_on_diff_spec.rb's "hides suggestion popover" example.

Reproduction

  1. Enable the rapid_diffs_default_on_mr_show feature flag.
  2. As a user who has never dismissed the suggestion popover, open a merge request's Changes tab.
  3. Hover a diff line and start a new comment. The onboarding popover appears.
  4. Click "Got it". The popover disappears.
  5. Reload the page and start another comment. The popover no longer appears.

Merge request reports

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