Single DTO for collaborator objects

* As per the API spec, the collaborator objects
have the same fields so it's more prudent to use a
single DTO class to represent both the RiverCollaborator
and BucketCollaborator entities.
Refactor rivers controller, service and DAO

* Removed @ResponseBody from RiversController#addFollower
The endpoint mapped to this method simply returns the HTTP
status code

* Modified the RiverService methods for managing
followers and collaborators to user AccountDTO
and CollaboratorDTO

* Refactored RiverDao#addCollaborator so that the
RiverCollaborator is instantiated and its properties set
within the method.

* Added a dateAdded property to the RiverCollaborator
entity

* Updated the controller integration tests to reflect the changes
to the endpoints and request body