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: https://secure.phabricator.com/D6247