Try to diagnose App Login only for OAuth providers which support it

Summary: We currently try to do "app login" for all OAuth providers, but not all of them support it in a meaningful way. Particularly, it always fails for Google.

Test Plan: Ran google diagnostics on a working config, no longer got a diagnostic failure.

Reviewers: btrahan, vrana, csilvers

Reviewed By: csilvers

CC: aran

Differential Revision: https://secure.phabricator.com/D2377