Add mail keys to Ponder questions

Summary:
We need to go slightly farther to stub reply handler functionality for Ponder in at least some configurations, where we rely on the presence of a unique random key to generate per-object or per-object+user reply addresses.

This should probably be formalized in an interface since it's currently pretty ad-hoc.

Test Plan:

• Made comments in Ponder under a per-user email configuration.
• Ran migration, verified mail keys were generated.
• Ran migration again (with --apply), verified existing questions were skipped.
• Created a new question, verified mail key generation.

Reviewers: pieter

Reviewed By: pieter

CC: aran

Maniphest Tasks: T1873

Differential Revision: https://secure.phabricator.com/D3665