[ns_api/upload] Re-format the response to uploading files.
[upload] Don't copy out return file information on file upload, already done by jsonification.
[auth] Add support for identifying when a provider is 'unknown' while still supporting eas fall-back for console authentication.

Summary: Provide support for positive identification of EAS vs unknown provider + fallback to EAS for console auth.

Test Plan: auth eas and unknown provider when changes are landed.

Reviewers: spang

Differential Revision: https://review.inboxapp.com/D276
[auth/unknown] Better handling of unknown providers.

Summary:
If we don't know what the provider is, we can still try to auth through
exchange (though don't show exchange logo for this) and if that fails
then gracefully tell the user that we don't support that provider.

Support for gathering emails at that point should be added at the front-end.

Refs: T213

Test Plan: review code.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D270