Originally, DiffFragmentQueueView's sole responsibility was to handle

the queueing and loading of fragments from the server and to inject

those fragments into the DOM. Over time, it's been expanded to handle

interaction on each fragment. This added to the complexity a bit,

requiring additional bookkeeping and expensive selectors that worked

across the whole page.
This change splits the per-fragment handling out of

DiffFragmentQueueView and into its own DiffFragmentView class. This

handles the hover controls and expansion/collapsing logic. The

mouse/click events and centered collapse button tracking are all handled

per-view now, allowing the event selectors and button bookkeeping to be

more efficient. The view now only keeps the CenteredElementManager

around when needed, and doesn't require a page-wide selector every time

there's an expansion/collapse.
Unit tests have been updated for DiffFragmentQueueView for some of the

new tracking, and new tests were added for DiffFragmentView.

Unit tests pass.
Manually tested that all hover, expansion, and collapsing logic all

worked correctly.