Rewrite the implementation of related tags to be simpler, faster, and
more accurate:
* The related tags are now calculated by taking a random sample of 1000
posts, finding the top 250 most frequent tags among those posts, then
ordering those tags by cosine similarity.
* Related tags can generally be calculated in 50-300ms at these sample
sizes. Very high sample sizes (25000+ posts) are still relatively fast
(1-3 seconds), but generally they don't improve accuracy much.
* Related tags are now cached in redis rather than in the tags table.
The related_tags column in the tags table is no longer used.
* Only the related tags in the search taglist are cached. The related
tags returned by the 'Related tags' button are not cached.
* The cache lifetime is a fixed 4 hours.
* The 'Related tags' button now works with metatags.
* The /related_tag page now works with metatags and multitag searches.
Fixes#4134, #4146.
Also fixes a bug where mod actions weren't logged on mass updates.
Creating the mod action silently failed because it was called when
CurrentUser wasn' set.
* Fix tests to run the searches for real instead of mocking everything out.
* Fix SavedSearch.populate to only use the read only database in
production because in breaks things in tests. Specifically:
the posts get created in one db connection but searched for in
another, but the second transaction doesn't see the uncommitted posts
in the first transaction, so the search doesn't work.
