Add ability to edit resolvability of a pending comment

Follow-up from discussion: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7376#note_106854334

When editing pending comments:

• It's important that the user can edit the “Resolve/unresolve discussion” checkbox. https://gitlab.com/gitlab-org/gitlab-ee/issues/7905 proposes to add that toggle to an already saved pending comment, but that functionality should still exist in the editing form, just like when adding a new comment.
• The bottom padding of the pending comment should be 16px and not 8px.
• Per https://gitlab.com/gitlab-org/gitlab-ee/issues/7905, the [Pending] Discussion will be resolved. header should be removed.

Current	Proposal