Single DTO for stripped down Account

* Replaced all the static inner Account classes in the DTO objects to
the AccountDTO class. This makes the DTO classes that return an
account object much cleaner, reduces redundancy and espouses
reuse

* Refactored GetCollaboratorDTO so that only the account and owner
information is returned. This is following the change that switches
the `(rivers/buckets)/:id/collaborators` to use the :account_id instead
of :collaborator_id

* Refs #45