Created a keyboard shortcut to add comments to a diff

Created a keyboard shortcut (r/R) to add comments to a block of a diff. Also
saved the chunk that is highlighted when navigating the diff using keyboard
shortcuts to use for detecting where to put the comments.

Testing Done:
* Navigated through a diff using keyboard shortcuts and pressed all three of
  the keyboard shortcuts to bring up a comment, which bring up the comment box.
* If no block is selected, nothing happens when the shortcuts are pressed.
* Tested on multi and single line blocks.
* Tested on a diff with two files.

Bugs closed: 44

Reviewed at https://reviews.reviewboard.org/r/5442/