Summary:
- Add an endpoint to allow joining an association
- Replace the "Join" button with a deactivated one if the user is already a member

Test Plan:
- Not connected, click a join button, check that you've joined after logging in
- Already logged in, click a join button, and check that it's deactivated afterwards

Reviewers: corbeaumax, Armteck

Reviewed By: corbeaumax

Maniphest Tasks: T65

Differential Revision: https://phabricator.atilla.org/D34