Add password authentication and registration to new registration

Summary:
Ref T1536. Ref T1930. Code is not reachable.

This provides password authentication and registration on the new provider/adapter framework.

I sort of cheated a little bit and don't really route any password logic through the adapter (instead, this provider uses an empty adapter and just sets the type/domain on it). I think the right way to do this //conceptually// is to treat username/passwords as an external black box which the adapter communicates with. However, this creates a lot of practical implementation and UX problems:

- There would basically be two steps -- in the first one, you interact with the "password black box", which behaves like an OAuth provider. This produces some ExternalAccount associated with the username/password pair, then we go into normal registration.
- In normal registration, we'd proceed normally.

This means:

- The registration flow would be split into two parts, one where you select a username/password (interacting with the black box) and one where you actually register (interacting with the generic flow). This is unusual and probably confusing for users.
- We would need to do a lot of re-hashing of passwords, since passwords currently depend on the username and user PHID, which won't exist yet during registration or the "black box" phase. This is a big mess I don't want to deal with.
- We hit a weird condition where two users complete step 1 with the same username but don't complete step 2 yet. The box knows about two different copies of the username, with two different passwords. When we arrive at step 2 the second time we have a lot of bad choices about how to reoslve it, most of which create security problems. The most stragihtforward and "pure" way to resolve the issues is to put password-auth usernames in a separate space, but this would be incredibly confusuing to users (your login name might not be the same as your username, which is bizarre).
- If we change this, we need to update all the other password-related code, which I don't want to bother with (at least for now).

Instead, let registration know about a "default" registration controller (which is always password, if enabled), and let it require a password. This gives us a much simpler (albeit slightly less pure) implementation:

- All the fields are on one form.
- Password adapter is just a shell.
- Password provider does the heavy lifting.

We might make this more pure at some point, but I'm generally pretty satisfied with this.

This doesn't implement the brute-force CAPTCHA protection, that will be coming soon.

Test Plan: Registered with password only and logged in with a password. Hit various error conditions.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, chad

Maniphest Tasks: T1536, T1930

Differential Revision: https://secure.phabricator.com/D6164