Don't show the expand icon if a header isn't expandable.

If multiple sections of collapsed context exist within a single function, the
headers listed in the collapsed sections will be the same function name, but
only the first one is expandable. Unfortunately, the "+" icon was still shown,
despite it not having an <a> tag around it.

Testing done:
Loaded a diff where I noticed this issue. Saw that the function name was still
shown, but the expand icon was gone.

Reviewed at https://reviews.reviewboard.org/r/5476/
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.

Testing done:
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.

Fixes bug 48.

Reviewed at https://reviews.reviewboard.org/r/5478/
Fix vertical alignment of the Commit field value.

The CSS for the span tags were too general, and were causing the value
for the Commit field to be too low. We now limit to the particular span
tags we care about.