R1:d24e66724d3b

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…