Per-group/role increase to users-per-role cap

[adehnert]

Several group roles have a cap --- President is capped at two, shared storage access at six, etc.. We sometimes let individual groups go over this cap --- the ASA, Tech Squares, etc. have more than six people with shared storage access, ESP wants more than two treasurers, some groups(?) have more than six financial signatories, etc.. We should be able to encode in the DB "this group can have $n>cap$ people, rather than just "this group currently has $n>cap$ people".

It's plausible that we should handle reducing the cap for the group as well as increasing it, though I'm not sure when we'd want to do that.

ASA-#84 is related in that it also requires per-group/role data (the list to sync), and a custom max-users-per-role cap should be respected in moira lists too.