Simplify filter_labels method in IssuableBaseService
Simplify filter_labels method in IssuableBaseService

IssuableBaseService::filter_labels() has been refactored to call
a new `label_ids_to_filter` method.

mmkassem/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq