[DONE] Groups and permissions refactor

[xoft]

So far I've come up with the following webadmin schema:

Tab Online players:
- (pretty much what STR_Warrior drew, without the Offline players section)
- per-world list of players
- name
- position (?)
- rank
- can change each player's rank
- can kick each player
- can view list of player's permissions
- can send PM
- special "Player details" subpage?

Tab Ranks:
- (more or less done in the PR)
- displays list of ranks
- add / rename / delete a rank
- add / remove groups to ranks
- set rank visuals
- view all players in a rank (?)

Tab Permissions:
- displays list of groups
- add / rename / delete a group
- add / remove permissions to groups

I've split the ranks / permissions into separate pages, because there will be possibly a lot of entries in each and two long lists on a single page would be impractical.