Edit merge request inline (and remove merge request edit web form)

• Similar to the issue page, remove the merge request edit web form and use an inline edit UI.
• Use a similar design to https://gitlab.com/gitlab-org/gitlab-ce/issues/24873.
• Remove the fields Assignee, Milestone, Labels, Weight, and Due Date from the inline UI.
• The Approvers/Approvals section, the Target branch field, and the checkboxes should remain in the inline UI.
• Ensure mobile still works.
• Do any technical clean up from removing merge request edit web form.