Add a cancel button, provider crumb, and account card to registration

Summary:
Ref T1536.

• Add a "Cancel" button, to get back to login.
• Add a crumb showing the registering provider.
• Add an account card when registering with an external account
• Tailor some language to make it less ambiguous ("Phabricator Username", "Register Phabricator Account").

Test Plan:
{F46618}

{F46619}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6213