Allow for Banning from freely joinable Groups

We’re looking at using Groups to allow access to semi-private Categories. These Categories are to be held to higher behavior & moderation standards than the primary Discourse.

We’d rather set the Groups to be self-joinable, but the Owners & Mods need to be able to ban people from the Groups if they don’t abide the ground rules.

(They would still have access to the common areas of the Discourse.)

Since we don’t see a “Ban” option, we’ll have to have the Owners manually administer the membership, as best we can tell, but a ban feature would let the rest of the membership mostly manage themselves.