R1:a75766c0e501

Replace old rate limiting in password login flow with "SystemAction" rate limiting

Summary:
Depends on D20667. Ref T13343. Password auth currently uses an older rate limiting mechanism, upgrade it to the modern "SystemAction" mechanism.

This mostly just improves consistency, although there are some tangential/theoretical benefits:

- it's not obvious that making the user log GC very quickly could disable rate limiting;
- if we let you configure action limits in the future, which we might, this would…