Add Mailtags to Paste

Summary: Fixes T7786. Adds very basic mailtag support.

Test Plan: Tested changing language, title to a Paste, didn't get notifications on my test account.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7786

Differential Revision: https://secure.phabricator.com/D12404