Add passthru to AphrontRequest

Summary:
For transaction interfaces, I want to prompt the user when they take an action that has no effect, e.g.:

Action Has No Effect

You can not close this task, because someone else has already closed it.

Do you want to post your comment anyway?

      [Cancel] [Post Comment]

We already do this for Differential, but it's all hard-coded. T912 is an open task for fixing this for Maniphest.

To do this in a general way, I want to embed the entire request in the dialog as hidden inputs, then add a "continue" key and resubmit the form. The endpoint will read this key the second time through and apply what effects it can (e.g., just post a comment).

This adds a mechanism for getting all the request data, minus "magic" like dialog and csrf. We need to jump through some hoops because of how PHP encodes arrays.

Test Plan: Ran unit tests, built "no effect" dialogs on top of this.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T912, T2104

Differential Revision: https://secure.phabricator.com/D4158