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
Provide more storage space for password hashes and migrate existing hashes to "md5:"

Summary: Ref T4443. Provide more space; remove the hack-glue.

Test Plan: Logged out, logged in, inspected database.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4443

Differential Revision: https://secure.phabricator.com/D8269
Implement bcrypt hasher, transparent login upgrade, and explicit upgrade for passwords

Summary:
Ref T4443.

  - Add a `password_hash()`-based bcrypt hasher if `password_hash()` is available.
  - When a user logs in using a password, upgrade their password to the strongest available hash format.
  - On the password settings page:
    - Warn the user if their password uses any algorithm other than the strongest one.
    - Show the algorithm the password uses.
    - Show the best available algorithm.

Test Plan: As an md5 user, viewed password settings page and saw a warning. Logged out. Logged in, got upgraded, no more warning. Changed password, verified database rehash. Logged out, logged in.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4443

Differential Revision: https://secure.phabricator.com/D8270
Allow hashers to side-grade hashes across cost settings

Summary:
Ref T4443. In addition to performing upgrades from, e.g., md5 -> bcrypt, also allow sidegrades from, e.g., bcrypt(cost=11) to bcrypt(cost=12). This allows us to, for example, bump the cost function every 18 months and stay on par with Moore's law, on average.

I'm also allowing "upgrades" which technically reduce cost, but this seems like the right thing to do (i.e., generally migrate password storage so it's all uniform, on average).

Test Plan:
  - Fiddled the bcrypt cost function and saw appropriate upgrade UI, and upgraded passwords upon password change.
  - Passwords still worked.
  - Around cost=13 or 14 things start getting noticibly slow, so bcrypt does actually work. Such wow.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4443

Differential Revision: https://secure.phabricator.com/D8271
Modernize VCS password storage to use shared hash infrastructure

Summary: Fixes T4443. Plug VCS passwords into the shared key stretching. They don't use any real stretching now (I anticipated doing something like T4443 eventually) so we can just migrate them into stretching all at once.

Test Plan:
  - Viewed VCS settings.
  - Used VCS password after migration.
  - Set VCS password.
  - Upgraded VCS password by using it.
  - Used VCS password some more.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4443

Differential Revision: https://secure.phabricator.com/D8272