Add templated links for group boards

The boards for my groups (groupcode review, groupdatabase, ~"group::editor", groupsource code) don't change from milestone to milestone, with the exception of the &milestone_title= portion of the URL query string. This merge request updates our monthly issue template so my group listings link directly to the board for that group, sans milestone. I've left stubs to indicate I should add in the planning issue for each group. It doesn't completely automate the process for me, but simplifies it somewhat.

Diana may or may not want to accept this work, but as part of reviewing the boards, I wanted to propose it nonetheless. I like avoiding repetitive work whenever possible.