Improve UI messaging around "one-shot" vs "session upgrade" MFA

Summary:
Depends on D19899. Ref T13222. When we prompt you for one-shot MFA, we currently give you a lot of misleading text about your session staying in "high security mode".

Differentiate between one-shot and session upgrade MFA, and give the user appropriate cues and explanatory text.

Test Plan:

• Hit one-shot MFA on an "mfa" task in Maniphest.
• Hit session upgrade MFA in Settings > Multi-Factor.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19900