R1:fded36cc2153

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

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