Add a "Diff Size" column to the dashboard.
This change adds a new column to our Review request datagrids that shows an

approximate size for the diff (added and removed lines). It doesn't correspond

very directly to what users will see in the diff viewer, but it should be good

enough to give a general sense of how big the patch is.

Added the column to my dashboard when running a dump of the

reviews.reviewboard.org database. Saw nice diff sizes for all changes that

included a diff, and nothing for attachment-only review requests.