Read POST data sightly earlier in request startup

Summary:
On instances, the "SiteSource" (for site config) pretty much copy-pastes the "read POST data" block because it needs to make some decisions based on POST data when handling inbound mail webhooks.

Move the upstream read a little earlier so we can get rid of this. Now that this step is separated and must happen before the profiler, there's no reason not to do it earlier.

Test Plan: POSTed some data across pages without issue, will remove duplicate code in upcoming change.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20073