Multiple issue ids in "Related to" filter
Multiple tickets can be specified in the "Parent Task" at custom query.
Feature #30482 Multiple issue ids in "Parent task" filter
Similarly, this patch enable multiple tickets to be specified in the "Related To" field.
(We (Tomoko Shimizu, Ko Nagase and Takashi Kato) created this patch in Japan Redmine Patch Meetup 30th (https://redmine-patch.connpass.com/event/274438/) event.)