Move diff expand/collapse support into the new DiffReviewableView.

The resulting code is actually cleaner than the old code, because we
have more context to work from, instead of having to embed a bunch of
stuff inside the links.

There's now unit tests that ensure our expand/collapse logic works
correctly.

It also fixes a regression in collapsing that was introduced in the
original DiffReviewable model that was pulled out early on in the
JavaScript refactoring.

This is the last work needed before we can move the DiffComment
support over.

Tested expanding in every possible way, and collapsing each expanded
state.

Unit tests pass.