Diff fragments were appearing in a broken semi-collapsed state in both

the review dialog and in entries that started as collapsed but were then

expanded. Since they couldn't perform height calculations (due to not

being visible on-screen), they had a sort of squashed look, requiring a

mouse-over to fix. The review dialog itself shouldn't have even allowed

for a collapsed state to begin with.
The diff fragment views can now be given default options through the

queue, currently used to control a new option indicating whether

collapsing is allowed. This option is off by default, and enabled only

for the review request page's entries. That fixes the display in the

review dialog.
The entries now keep track of their rendered diff fragments, through a

series of new callbacks in the queue. When expanding an entry, the diffs

are forced into a collapsed state, so they'll appear correctly.

Tested that diff fragments behaved correctly for initially-expanded

entries, initially-collapsed entries, and when repeatedly collapsing and

expanding entries.
Tested diff fragment display in the review dialog.
Tested the above behavior with entry collapse states after expanding

context in a diff.