Move a lot of pre-request checks to PhabricatorStartup

Summary:
We have a lot of mess to get through before we can load libphutil and enter Phabricator code properly. Move it to a dedicated class.

I'm probably going to merge PhabricatorRequestOverseer into this, although the check that lives there now is kind of weird. It also does not really need to be a pre-load check and could be handled better.

I stopped shoving stuff in here once I got to ENV stuff, I'm going to tackle that next.

Test Plan: Ran phabricator normally; introduced fatals and misconfigurations. Grepped for changed symbols.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran, asherkin

Maniphest Tasks: T2223

Differential Revision: https://secure.phabricator.com/D4282