R1:15cc475cbd8c

When a comment was signed with MFA, require MFA to edit it

Summary:
Ref PHI1173. Currently, you can edit an MFA'd comment without redoing MFA. This is inconsistent with the intent of the MFA badge, since it means an un-MFA'd comment may have an "MFA" badge on it.

Instead, implement these rules:

- If a comment was signed with MFA, you MUST MFA to edit it.
- When removing a comment, add an extra MFA prompt if the user has MFA. This one isn't strictly required, this action is just very hard to undo and…