Move Facebook setup checks to post-install

Summary: Technically we should have these for all the OAuth providers but I don't think anyone really has trouble with them and it can probably be done generically after T1536. Preserve the functionality, at least.

Test Plan: Broke my config, verified warnings appeared.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2228

Differential Revision: https://secure.phabricator.com/D4506