Make AphrontProxyResponse reduce to a real response instead of building a string

Summary: Currently, AphrontProxyResponse is expected to build a string. This prevents some response types (like Dialog) from being proxied, because they have special rules. Instead, make proxy responses reduce into a non-proxied response so it's possible to proxy any type of response and hit all the normal rules for it.

Test Plan: Built a proxied DialogResponse on top of this.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2104, T912

Differential Revision: https://secure.phabricator.com/D4159