Migrate unified diff screen from prettify to CodeMirror, reusing as much as
possible from the side by side diff view.
This now builds and mostly works.
TODO:
1. Maybe implement proper behavior for range based comments.
- Currently, pressing 'c' with text selected does not open a comment.
2. Support hiding line numbers in Unified.
3. Factor out more common code, minimize duplication.
- UnifiedCommentManager and SideBySideCommentManager.
- UnifiedSkipBar and SideBySideSkipBar.
These will be addressed in follow-up commits.
Change-Id: I91935613510879436a911ff1dcc191c729f0f06d
f156a4d76a