Prevent `{{title}}` from being used without any prefix in the summary item

The following discussion from !300 (merged) should be addressed:

• @godfat-gitlab started a discussion: (+4 comments)

  Technically, if one uses {{title}} (without leading - ) then it is still subject to performing a quick action.

  It's not clear to me how to "escape" quick actions though. Maybe we can insert a leading space? This ways it doesn't change the format too much, and it'll invalidate the quick action.