Consolidate environmental initialization

Summary:
We have a bunch of code duplication now between __init_script__.php and webroot/index.php. Consoldiate these methods and move them into PhabricatorEnv.

Merge PhabricatorRequestOverseer into PhabricatorStartup.

Test Plan: Loaded page, ran script. Wiped PHABRICATOR_ENV; loaded page, ran script; got errors.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2223

Differential Revision: https://secure.phabricator.com/D4283