R1:433c6550b235

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…