Convert "Rename User" from session MFA to one-shot MFA

Summary:
Depends on D20035. Ref T13222.

• Allow individual transactions to request one-shot MFA if available.
• Make "change username" request MFA.

Test Plan:

• Renamed a user, got prompted for MFA, provided it.
• Saw that I no longer remain in high-security after performing the edit.
• Grepped for other uses of PhabricatorUserUsernameTransaction, found none.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D20036