Add Experiment: Edit response body

[scottjehl]

This experiment allows you to edit the response of any same-domain file requested in this test. The text in this field will replace the text of the response before it is sent to the browser.

As an initial release, allowing editing the initial request response body is likely the most important one (the HTML).
Ideally, we'd allow one or many of these fields and offer all same domain requests in the select menu.

Proposed Design

[scottjehl]

Realized this morning that there's probably a handy way for us to post bigger response body swaps in experiments. Since the form fields post with the experiment, they're written to the test metadata, so I think as long as the experiments worker has the id of the current test, it can fetch that result's json (which won't have much populated in it during the experiment), and get the encoded text that was posted with the experiment for a given request.

Building on that, I think this will be able to work from the start test page too once we move experiments there, but it'll need to be different field types: an freeform input for the request to replace (any substring match should work), and an unpopulated textarea since we won't know the content yet.

[scottjehl]

Merged!