R1:abc030fa008b

Move account passwords to shared infrastructure

Summary:
Ref T13043. This moves user account passwords to the new shared infrastructure.

There's a lot of code changes here, but essentially all of it is the same as the VCS password logic in D18898.

Test Plan:
- Ran migration.
- Spot checked table for general sanity.
- Logged in with an existing password.
- Hit all error conditions on "change password", "set password", "register new account" flows.
- Verified that changing password logs out other sessions.