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 Password" panel. Successfully changed password.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13043

Differential Revision: https://secure.phabricator.com/D18902