Transactions - add infrastructure for "mentions"

Summary: Fixes T4036. Now if you say something on diff X like "This reminds me of Tx and Dy and commitHashFoo and Px." each of those objects gets a little visible transaction that the mention occurred. No feed, email, or notifications.

Test Plan: made a comment like above and verified transactions. also submitted a diff that "Fixes Tx" and Tx did not get the transaction as expected.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: joshuaspence, epriestley, Korvin

Maniphest Tasks: T4036

Differential Revision: https://secure.phabricator.com/D10451