Disabling a user should kick them out of all groups

[jmallach]

I am not 100% certain, but I suspect there's a bug in which [deactivated user]s that are members of a group are summoned to votes.

While this in itself is probably a bug, I don't think this condition should ever happen: when a user is deactivated, I think it should be disowned of any group membership.

Additionally, if they have pending invites in any thread or poll, they should be uninvited to vote, in order to have the voting results not affected by users that won't be able to vote on a poll, anyway.

cc: @anarute