Add an activity indicator for AJAX calls.

This activity indicator is triggered during AJAX operations such as
fetching the review form, marking a change as submitted, saving
comments, or saving parts of review requests. This appears in the
top-right of the page in a simple but eye catching little box,
regardless of where you are on the page.

Tried saving data in review requests, making comments on diffs and screenshots, bringing up the review dialog, and marking changes as submitted, discarded, and reopening reviews. Saw the "Loading..." or "Saving..." indicator as appropriate.