Move some useful code for the diff viewer UI into central locations.
This takes some of the stuff from the diff viewer that my upcoming

text-based review UI change needs and moves them into more useful

places.
The lesscss definitions in diffviewer.less have been moved into

defs.less, making it easier to match up styles.
The multi-line comment selector for the diff viewer tables has also been

pulled out into a new RB.TextCommentRowSelector class, which other

review UIs can trivially use.

Tested along with my text-based review UI change.
The diff viewer still looks correct, and I can comment on lines.