move Rules event invocation to a helper method in flag_flag

The clean-up work in #2202969: clean up flag() and its low-level helpers is great, but flagging_insert() is now a bit on the big side.

I think it would make sense to move the Rules stuff out to a helper method, so replace this:

    // Invoke Rules event.
    if (module_exists('rules')) {
      $event_name = 'flag_flagged_' . $this->name;
      // We only support flags on entities.
      if (entity_get_info($this->entity_type)) {
        $variables = array(
          'flag' => $this,
          'flagged_' . $this->entity_type => $entity_id,
          'flagging_user' => $account,
          'flagging' => $this->get_flagging($entity_id, $account->uid),
        );
        rules_invoke_event_by_args($event_name, $variables);
      }
    }

with something like this:

    // Invoke Rules event.
    if (module_exists('rules')) {
      $this->invoke_rules_event(ACTION, $flagging, whatever other params we need);
    }

and let the helper method do the work of building up the stuff that needs to be passed to Rules.

While the goal of #2202969: clean up flag() and its low-level helpers was to remove tangled helper methods, I think that this is a good candidate for a helper: it's a single operation, which requires quite a few lines of code, and which is perfectly described by the method call to invoke_rules_event(). It's also code that occurs twice at the moment.

Postponed because #2250543: pointless call to get_flagging() needs to get in first.