Commit of partial work (this commit will be amended) to share with J...

...ames

        - during validation stage:
            Event 1: resource_valid
                - run on each Module in URL path
                - Does this event need a better name, like validate_resource ?

                - binders:
                    - cb_users_*
                        - load user_id into context
                    - cb_accounts
                        - load account_db/accout_id into context

            Event 2: validate
                - only run on last Module, like resource_exists

        If only resource_exists() method took Context as parameter, could fold o
`UserId` should be a valid doc ID, so use `couch_mgr:open_cache_doc/2` to fetch it and return a 404 error in cb_context (`crossbar_util:response_bad_identifier/2`) you get an {'error', 'not_found'}. Set user id on success. `response_db_fatal` otherwise? Something like that should be sufficient.