Add user to Collective

[danselden]

Process by which an admin of a collective can invite a particular user to a collective. User is notified of invitation and can choose to accept or reject participation, and to add their mikroacts to the collective's listing.

[supervacuo]

• [x] add user who is already following the collective
• [ ] invite users by email / username

[supervacuo]

All backend adding-member stuff is done. if you click "Add members" from the actions drop-down on a collective you manage, it'll give you a checkbox list of users who a) follow that collective and b) aren't members already. ticking them and clicking "add.. " adds them to the member list.