R1:9c00a437848c

Add a more modern object for storing password hashes

Summary:
Ref T13043. Currently:

- Passwords are stored separately in the "VCS Passwords" and "User" tables and don't share as much code as they could.
- Because User objects are all over the place in the code, password hashes are all over the place too (i.e., often somewhere in process memory). This is a very low-severity, theoretical sort of issue, but it could make leaving a stray `var_dump()` in the code somewhere a lot more dangerous than it…