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 set up one of apache, nginx or lighttpd.
  • Tweaked a few things I've seen users have difficulty with.

This should have no effect on any existing installs, but make the process much simpler for future installs.

Closes T2221.
Closes T2223.
Closes T2228.

Test Plan:

• Removed my PHABRICATOR_ENV and went through the install process.
• Generated and read documentation.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2221, T2223, T2228

Differential Revision: https://secure.phabricator.com/D4596