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.
  • Registered a new account.
  • Changed password.
  • Switched back to master.
  • Logged in / out, changed password.
  • Switched back, logged in.
• Ran unit tests (they aren't super extensive, but cover some of the basics).

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, kofalt

Maniphest Tasks: T4443

Differential Revision: https://secure.phabricator.com/D8268