[core] Add bitwise privilege tracking

Includes tracking who is in a channel and IRCv3 multi-prefix support.
I'm leaving #312 open, though, until I'm sure I like the way the API
for the new IRCv3 features works.
[rss] Fix missing import
[adminchannel, clock, radio] Use new channel privileges

Close #326
[docs] Deprecate old-style permissions stuff