Favorite groups

[ToksT]

implements #635

• Basic members can have 3 groups, gold can have 5, platinum can have 10 (arbitrarily decided)
• Each favorite group can have up to 10,000 posts (arbitrarily decided)
• Favorite groups can be viewed by anyone but only edited/deleted by their creator
• Can add posts to favgroups like pools: either edit the list of ids or type favgroup:[name or id] in the post's tags
• There is a favgroup:[id] search metatag to allow advanced searches. This is implemented with posts.id IN, which I would have expected to be slow for large favgroups, but is surprisingly fast. Not slow even with thousands of posts in the favgroup.
• Can browse through favgroups like pools
• There's a link to your own favgroups in the subnavbar, and to other people's via their profile

[r888888888]

Only concern is this table blowing up in size but it probably won't be as bad as favorites. For that reason I think being more aggressive with the limiting is probably a good idea, can always raise them later if usage isn't high.