Share tags2

Supersedes #11351.
This one's actually a good example. So far, Tags have been cached, and at this point, with $sync=true would've been persisted to the DB. Following @Raydiation's suggestion, I removed the caching which he thought was adding unnecessary complexity while possibly not speeding up things that much or at all (as we were sorting cached tags using `usort` before output, i.e. every time `getTags()` was called). Now, `getTags()` just reads them from the DB, using `ORDER BY`.