Rename collaborator property to is_collaborator

* If a user is collaborating on a river/bucket, that
does not make them an owner.

* TODO: Include read_only property in the response with
the collaboration info
Clear separation of assets

* Distinguish between the items that a user owns and those
they are collaborating on.
Add "Uncollaborate" button on dashboard

* The "uncollaborate" button on the content page gives users
a way remove themselves from the list of collaborators of
rivers/buckets (they are collaborating on)