[MNG-1819] Add an epic description template
This adds a template to use for new epics, largely based on the old template that we used to have, plus the new checklist to incorporate project management best practices from !197 (comment 460315402)
This adds a template to use for new epics, largely based on the old template that we used to have, plus the new checklist to incorporate project management best practices from !197 (comment 460315402)