Check permissions before deleting river

* A river can only be deleted by its creator.
Fix read/unread drops query

* Where the isRead filter parameter has been specified, wrap the
result set of the main query in a temp table and filter the result set using
the `drop_read`  column
Filter bucket drops by state

* The state parameter takes on one of two values: "read" and "unread"
and is used to distinguish between read and unread bucket drops