Add empty 'release note' linter for commit message

Still a warning, but better than letting these slip through.

Related story:
  https://www.pivotaltracker.com/story/show/48023785 (delivery)

Change-Id: I45e6fdb7dfa64ba2e5e6e9b6444cb51fb87e13ee
Reviewed-on: https://gerrit.causes.com/22211
Tested-by: Aiden Scandella <aiden@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>