Implement Controller and route to Accept/Reject Collab invitation

[BuddyLongLegs]

A single controller in which a user can accept or reject an invitation, on accept change the collab status to accepted and on reject, delete the collab object from the database. Just make sure that the email of req.user is same as the invited collaborator and project.

Also create a route to get emails and statuses of collaborators of a project.

[chandra1899]

claim