Fixes #6167 - Make sure user has access to river object and target.

This is a WIP fix that joins river object and target to entities table and makes sure user has access to both.

The default elgg_get_access_where_sql() is used to generate the where clauses. Therefore it also checks whether the entities are enabled.

If this approach proves to be sufficient it makes the river access_id needless. Therefore some additional modifications are needed before merging.

(This PR becomes more meaningful once discussion replies are migrated to entities, so it might be best to deal with #6177 first.)