Give the phabricator command to run to delete repos.

Summary:
We didn't do this before because the phabricator command takes the
callsign, and we didn't have the callsign handy at delete-message
time.  But I've rewritten the code to pass the necessary data around,
so now we can say:

Repository https://khanacademy.kilnhg.com/Code/Website/Group/topics-refactor has been deleted: run (on toby):
   PHABRICATOR_ENV=khan phabricator/bin/repository delete TR

Test Plan:
Manually ran the script, and verified the output (I had lots of repos
to delete after the Great January Kiln Cleanup).

Reviewers: benkomalo

Reviewed By: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D2004