Improve more crumbs and cancel buttons for auth

Summary:
Ref T1536.

• When linking accounts after initially failing, make the crumb say "Link Account" instead of "Login".
• When on the LDAP failure form, show a "Cancel" button returning to start (if logging in) or settings (if linking accounts).
• Allow providers to distinguish between "start", "login" and "link" rendering.

Test Plan: Linked and logged in with LDAP and other registration mechainsms.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6214