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 seems reasonable to MFA.

Test Plan:

• Made normal comments and MFA comments.
• Edited normal comments and MFA comments (got prompted).
• Removed normal comments and MFA comments (prompted in both cases).
• Tried to edit an MFA comment without MFA on my account, got a hard "MFA absolutely required" failure.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20340