Apply group label to new retro issues

This fixes a bug introduced in !241 (merged) that caused all groups sharing a retrospective project to be pinged on the single most recently-created retrospective issue each month. The problem was that the issues did not have team labels, so the most recently-created issue was being used by default. This adds a quick action to the default issue template, so at issues can now be matched with their group. Basically, this was the missing piece from !241 (merged).

Related to #36 (closed)