Make paths and Aphlict instance names less ambiguous

Summary:
Fixes T10783 (what little of it remains). Ref T10697.

Aphlict currently uses request paths for two different things:

• multi-tenant instancing in the Phacility cluster (each instance gets its own namespace within an Aphlict server);
• some users configure nginx and apache to do proxying or SSL termination based on the path.

Currently, these can collide.

Put a "~" before the instance name to make it unambiguous. At some point we can possibly just use a GET parameter, but I think there was some reason I didn't do that originally and this sequence of changes is disruptive enough already.

Test Plan: Saw local Aphlict unambiguously recognize "local.phacility.com" as instance "local", with a "~"-style URI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10697, T10783

Differential Revision: https://secure.phabricator.com/D15705