Now that we're showing more than review requests in the search results,
we need to give users the option to filter those results.

There's now a filter bar above the search results, showing a tab-like
view of All Results, Users, and Review Requests. Clicking one of these
will re-search with a filter applied.

Users can also combine filters in the URL, if they want, though there's
no UI for doing so. It's an advanced feature that may be more useful
down the road.

While here, I removed some old, dead code from the template and context
variables, and tweaked the number of pages shown in the paginator.

Tested searching with each filter type, with no filters, and with

combinations of filters. I always got the results I expected.