Move line commenting and chunk selection functionality into DiffReviewerView.

The majority of code that lived in $.fn.diffFile has been moved into
the new reviewable.

All the code that handles line selection is in a CommentDiffSelector
object, which listens for events on the diff reviewable's element.
This keeps the main reviewable much cleaner and easier to work with.

This also brings back the ghost comment flag, which disappeared in 1.7 (due to
a CSS rule change). That'll probably get backported to 1.7.

Tested commenting on single lines.

Tested commenting across multiple lines.

Tested clicking on comments.

Tested expanding comments.

Tested clicking on chunks, causing highlights.

Tested anchors to lines in the diff viewer.

Saw the ghost flag.