API does not allow joining/quitting a shake

[spaceninja]

On MLTSHP.com you can join and quit a shake. The API does not support this functionality.

[spaceninja]

Suggestion: Add the following endpoint (Since you can't follow a user, only the user's shake, this can be solved with one endpoint):

`POST /api/shake_id/(int: shakeid)/join

(this should also be available on the /api/shake_name and /api/shake_user endpoints.)

Joins the given shake as the authorized user. The user will then have access to post to this shake.

Parameters

• shakeid – the shake’s numeric ID

Status Codes

• 200 OK – the shake was joined, and the response is the followed shake
  • If the shake was already joined, then it should be unjoined. (Question: what should be returned if a shake is unjoined?)
• 400 Bad Request – the shake could not be joined
• 403 Forbidden – the shake could not be joined due to permission issues
• 404 Not Found – no such shake