R1:3c9153079f13

Make password hashing modular

Summary:
Ref T4443. Make hashing algorithms pluggable and extensible so we can deal with the attendant complexities more easily.

This moves "Iterated MD5" to a modular implementation, and adds a tiny bit of hack-glue so we don't need to migrate the DB in this patch. I'll migrate in the next patch, then add bcrypt.

Test Plan:
- Verified that the same stuff gets stored in the DB (i.e., no functional changes):
- Logged into an old password account.
- Changed password.