Add Disqus, Facebook, Google, GitHub auth config; AWS config

Summary: Also improve behavior for the "unknown config" warning.

Test Plan: Looked at configs, went through unknown config workflow.

Reviewers: btrahan, codeblock

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4310