Added password hashing API

Deprecated the old User::crypt, et. al password hashing
system and implemented an extensible password hashing
API.

The new Password class allows registering of child classes
and provides factory functions for creating new Password
objects. The built-in hash types are the old MediaWiki MD5
types, which are for backwards-compatibility only, and bcrypt.

Also included is support for wrapping existing hashes as well
as encrypting passwords with a configured encryption key.

Bug: 54948
Bug: 28419
Change-Id: I0a9c972931a0eff0cfb2619cef3ddffd03710285