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
