Change the way we do board permissions

Related to #479 (closed):

twiggers: Somewhat related: I'd rather attach the board to a regular Django group than edit their (committee)group's permissions each year: it's much more error-prone to re-add those permissions than it is to add and remove some members from a Django Group.

sversteeg: We could create a connection between committees and groups? I don't believe opening 12 user profiles and changing the groups is less error-prone than removing and re-adding the permissions. Maybe continue this is another issue, since I finished this one.

twiggers: The difference is the kind of mistakes you make. When opening/changing 12 user profiles, you may forget one. When manually adding the tens of permissions we need to give to boards, we're bound to forget one. This is followed by a board member then trying to do something they need to do, and then assigning themselves superuser privileges, and then later perhaps breaking the site.

Good point, needs discussion