[entity] add a new `cw_linkable_rql` method
* the new method returns the entities that are or may be related to the
current one by the relation passed as an argument.
Closes #3738011.
for table, column in [
('transactions', 'tx_time'),
('tx_entity_actions', 'tx_uuid'),
('tx_relation_actions', 'tx_uuid')]:
repo.system_source.create_index(session, table, column)
commit()