Refactor Phabricator mention rule to do data fetching in post processing

Summary:
This accomplishes two goals:

• Data fetching is now grouped across blocks.
• Demonstrates that D737 actually works.

Test Plan: Used @mentions in Phriction preview, they rendered properly. Verified
only one service call was being made across blocks.
Reviewed By: jungejason
Reviewers: hunterbridges, jungejason, tuomaspelkonen, aran
CC: aran, jungejason, epriestley
Differential Revision: 738