Share more HTTPSink code

Summary:
In the past, we did some additional magic on $response_string (adding profiling headers? Or DarkConsole?), so we could not share the pathway with HTTPSink. We no longer do this; share the pathways.

Also remove error handler initialization (duplicated in PhabricatorEnv), and move $sink initialization earlier. My general goal here is to allow PhabricatorSetup to emit a normal Response object and share as much code as possible with normal pages.

Test Plan: Loaded page.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2228

Differential Revision: https://secure.phabricator.com/D4285