Fix z-index of updates bubble.
Review Request #7569 — Created Aug. 5, 2015 and submitted
The updates bubble didn't have a z-index set, which could make it appear under
other elements. This wasn't immediately obvious before because it is the last
element in the body, so it did appear on top of most things.
Set up things to get an updates bubble, and then verified that it no longer
displayed behind file attachment captions.
Tool: PEP8 Style Checker Ignored Files: reviewboard/static/rb/css/pages/reviews.less Tool: Pyflakes Ignored Files: reviewboard/static/rb/css/pages/reviews.less