Fix: synchronize the asset lists in header and content view

* When an asset was added/deleted, the listings in the content view
(in the user dashboard) and header modals were not synching.
New parameter querying_account Service_Account::get_buckets & Service_Account::get_rivers

* The $querying_account parameter ensures that the is_owner and is_collaborator
properties are set with respect to the account that is viewing a user profile
Delete river

* API and service methods for deleting a river.

* Set the url property of the RiverList collection in assets.js to
point to Controller_User::action_rivers