Request a Membership for a Group based on the Group's id

[j1jlee]

Request a Membership for a Group based on the Group's id

Request a new membership for a group specified by id.

• An authenticated user is required for a successful response
• New membership exists in the database after request
• Success response includes the groupId, memberId, and a status of
  "pending"
• Error response with status 404 is given when a group does not exist with
  the provided id
• Error response with status 400 is given when the current user already has
  a pending membership for the group
• Error response with status 400 is given when the current user already has
  an accepted membership for the group