Generalize login flows for new registration

Summary:
Ref T1536. None of this code is reachable.

`PhabricatorAuthLoginController` provides a completely generic login/link flow, similar to how D6155 provides a generic registration flow.

`PhabricatorAuthProvider` wraps a `PhutilAuthAdapter` and glues the generic top-level flow to a concrete authentication provider.

Test Plan: Static only, code isn't meaningfully reachable.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6159