R1:1834584e98b6

Provide contextual help on auth provider configuration

Summary:
Ref T1536.

- Move all the provider-specific help into contextual help in Auth.
- This provides help much more contextually, and we can just tell the user the right values to use to configure things.
- Rewrite account/registration help to reflect the newer state of the word.
- Also clean up a few other loose ends.

Test Plan: {F46937}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision…