R1:b8a515cb29d8

Bring new password validation into AuthPasswordEngine

Summary:
Ref T13043. We have ~4 copies of this logic (registration, lost password recovery, set password, set VCS password).

Currently it varies a bit from case to case, but since it's all going to be basically identical once account passwords swap to the new infrastructure, bring it into the Engine so it can live in one place.

This also fixes VCS passwords not being affected by `account.minimum-password-length`.

Test Plan: Hit all errors in "VCS…