Add db_query signal and signal_query method

[OmeGak]

Add a db_query signal that allows a handler modify the query object. The signal_query method is expected to be used for sending the signal in order to guarantee that only one handler modifies the query.

An example of usage is the following:

query = (Registration.query.with_parent(self.event)
         .filter(Registration.is_state_publishable, ~RegistrationForm.is_deleted)
         .join(Registration.registration_form)
         .options(subqueryload('data').joinedload('field_data'),
                  contains_eager('registration_form'))
         .signal_query('publishable-registrations-in-participant-list'))

On the signal handler side, the query can be intercepted like this:

@signals.core.db_query.connect_via('publishable-registrations-in-participant-list')
def _intercept_publishable_registrations_in_participant_list(sender, query, **kwargs):
    return query.filter(...)