Consolidate and fix the expand diff links.

There's now a couple template tags for rendering the diff links in a
simpler way. This ends up going through a single utility template to
render each link.

As part of this, we now have a smarter place to handle the diff headers.
We no longer show expansion links next to headers showing
functions/classes that are not contained within that collapsed region.
The headers are still shown, but you can't expand.

Tested all the expand and collapse links.

Checked all headers across various diffs to make sure that the only expandable
ones were those that contained the function/class within the collapsed region.