Formalize configuration sources and source stacks

Summary: Currently, we have a configuration stack for unit tests, but they're built in to PhabricatorEnv. Pull them out and formalize them, so we can add more configuration sources (e.g., database).

Test Plan: Ran unit tests, web requests, scripts. This code had fairly good existing test coverage.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2223, T2221

Differential Revision: https://secure.phabricator.com/D4284