Allow "bin/auth recover" to generate a link which forces a full login session

Summary:
Depends on D19902. Ref T13222. This is mostly a "while I'm in here..." change since MFA is getting touched so much anyway.

Doing cluster support, I sometimes need to log into user accounts on instances that have MFA. I currently accomplish this by doing bin/auth recover, getting a parital session, and then forcing it into a full session in the database. This is inconvenient and somewhat dangerous.

Instead, allow bin/auth recover to generate a link that skips the "partial session" stage: adding required MFA, providing MFA, and signing legalpad documents.

Anyone who can run bin/auth recover can do this anyway, this just reduces the chance I accidentally bypass MFA on the wrong session when doing support stuff.

Test Plan:

• Logged in with bin/auth recover, was prompted for MFA.
• Logged in with bin/auth recover --force-full-session, was not prompted for MFA.
• Did a password reset, followed reset link, was prompted for MFA.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19903