Move target branch selection out of merge request edit web form

• Currently, to change the target branch of an existing merge request, you have to go to the edit screen and change it there.
• As part of the effort to remove the edit web form of a merge request, take the target branch selection UI and put it in the merge request sidebar or somewhere else, per comments: https://gitlab.com/gitlab-org/gitlab-ce/issues/40680#note_49548342.

Other components

• https://gitlab.com/gitlab-org/gitlab-ce/issues/36241 will make the title and description inline.
• Approvals UI need to be re-designed: https://gitlab.com/gitlab-org/gitlab-ee/issues/4190.
• Assignee, milestone, and labels don't need to be in the existing edit web form, since they are already in the sidebar.