Add searching, filtering, paging on user admin page
WIP: ToC UI mockup. Feature-toggled; disabled in prod.
Use Last-Modified instead of Etag to control rsolr caching

When using Etag while also load balancing requests between two (or more)
Solrs, we will get far to many cache misses, since the etag (for
identical requests) will be different between Solrs (since the etag is
calculated from the index version).

By using Last-Modified, as soon as we have cached the response from the
last opened searcher (in practice: the Solr that was commited to last or
restarted last) we will get cached hits on subsequent requests.