R1:7e17acfb68da

Remove PhabricatorSetup and make PHABRICATOR_ENV optional

Summary:
- PHABRICATOR_ENV is now optional. If you don't specify it, we won't load a config file.
- PhabricatorSetup is now gone.
- I removed the alternate file domain check for now, see T2380.
- `phabricator.setup` config is now gone.
- Rewrote documentation:
- No more mentions of `phabricator.setup`.
- Normal install guide no longer mentions PHABRICATOR_ENV. This is now an advanced topic.
- Clarified that you only need to…