Allow any transaction group to be signed with a one-shot "Sign With MFA" action
This is a one-shot gate and does not keep you in MFA.
- Used "Sign with MFA", got prompted for MFA, answered MFA, saw transactions apply with MFA metadata and markers.
- Tried to sign alone, got appropriate errors.
- Tried to sign no-op changes, got appropriate errors.
Reviewed By: amckinley
Maniphest Tasks: T13222
Differential Revision: https://secure.phabricator.com/D19897