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