Create Collection Membership model

[mononoken]

Currently, membership is recorded in a group through the accepted_at column in collection_invitations.

However, it feels now that we need a Membership model and table as well. These are separate things. While tracking accepted_at in invitations does not feel completely wrong, I want to now add a role for controlling user permissions in joined collections. This role does not belong on the invitations.

collection_memberships

• user_id: references (users)
• collection_id: references (collections)
• role: integer

[mononoken]

62284fab566020f1bb3ee648848c5ad6cfb9c8c1 implements Invitation as a separate model. We still need to make accepting invitations create Memberships though.

[mononoken]

c20c819c33dde9be96bec521ae5ad97c88d3b614 fixes by implementing accepting invitations. Users can now join other's collection.

49 needs to be addressed next so that user's can destroy their membership to a collection.