Allow MFA factors to provide more guidance text on create workflows

Summary:
Depends on D20016. Ref T920. This does nothing interesting on its own since the TOTP provider has no guidance/warnings, but landing it separately helps to simplify an upcoming SMS diff.

SMS will have these guidance messages:

• "Administrator: you haven't configured any mailer which can send SMS, like Twilio."
• "Administrator: SMS is weak."
• "User: you haven't configured a contact number."

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T920

Differential Revision: https://secure.phabricator.com/D20017