Fix review request caching when issue statuses change.

Our review request page's etags didn't take into account any state
changes caused by when issues were opened/closed. That's because we
didn't have anything we updated under those cases.

We're now repurposing the ReviewRequest.last_review_timestamp, which
was no longer used. This has been renamed last_review_activity_timestamp
in order to be a little more clear, and has a comment describing what it
takes into account.

Unit tests were added to make sure this doesn't regress.

Started with the unit test, which failed originally, showing that the
etags matched before and after changing the issue status.

After the change, the unit test passed.

I also reproduced it through the browser. Originally, if you opened two
tabs to the same page, closed the issue in one, and then entered the URL
again on the second, the issue would appear opened. After this change,
that doesn't happen, and the new load shows the correct state.