View merge request when source branch or target branch or both deleted

• Merge request widget should update in real-time when a branch is deleted.
• Tabs should update in real-time when a branch is deleted.

	Source present, target present	Source deleted, Target present	Source present, Target deleted	Source deleted, Target deleted
Open	Widget, view 4 tabs	Widget, view 4 tabs	Widget: view branch names, view exception message, view discussion tab only	Widget: view branch names, view exception message, view discussion tab only
Merged	Widget, view 4 tabs	Widget, view 4 tabs	Widget: view branch names, view exception message, view discussion tab only	Widget: view branch names, view exception message, view discussion tab only
Closed	Widget, view 4 tabs	Widget, view 4 tabs	Widget: view branch names, view exception message, view discussion tab only	Widget: view branch names, view exception message, view discussion tab only