Differential - add ability to setup "create" addresses for revisions

Summary: Fixes T1476. The body of the email should be just the output of some diff command.

Test Plan:
git diff master > text.txt; ./bin/mail receive-test --to <configured-diff-create-address> < text.txt; a diff was successfully created...! email generated had a working link to the diff.

./bin/mail receive-test --to <configured-diff-create-address> < README.md; a diff was not created as expected...! email generated had a sensical error message, telling me that the mail body should have been generated via a diff command

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: johnny-bit, Korvin, epriestley

Maniphest Tasks: T1476

Differential Revision: https://secure.phabricator.com/D11574