Fix expanding the top-most chunk of a file.

The logic for handling chunk expansion was doing a truthiness check on
chunk_index, instead of comparing it to None, preventing chunk 0 from
expanding properly. We now compare to None, like in all other cases.

Fixes bug #3026.
Merge branch 'release-1.7.x'
Fixed some visual issues with the chunk highlighter and diff margins.

Upon loading the diff viewer, the chunk highlighter would appear as a
little square in the bottom-left of the page, taking up space. This was
caused by an initial highlight call on the diff_index, which it wasn't
really able to highlight, so it fell back to being in a default
position. We no longer try to highlight nothing.

When it was highlighting, it wouldn't track the element it was supposed
to highlight if elements above it changed size. This was particularly
noticeable with image diffs. The highlighter would stay in its prior
position.

This was due to the highlighter living in the #diffs element, but now it
lives in the .diff-container. To make this work, I had to separate out
the rules and responsibilities of diff-container into diff-container and
diff-box elements.

While here, I fixed the margins between diffs so they're consistent
throughout the page.
Change the min-height on labels to be 16px (from 15px).

Our edit icon is now 16px tall, not 15. Trivial change.