Decide how to support page geometry etc. in tests and tuners

[erikrose]

Partway through implementing #18, I noticed that it takes more than simply HTML to reconstitute the geometry of a page. That is, a mere headless browser does not suffice to deliver the additional signal we wanted. Let's choose a solution.

See https://github.com/mozilla/fathom/wiki/PageGeometryCaptureSolutions for a more editable, history-tracking place to do it.

• See if there are any off-the-shelf libs out there that "freeze-dry" a page, preserving node geometry without saving every resource in its entirety. [Nope, Swathi couldn't find any.]

[erikrose]

We're going to break product work into its own repo so we can bloat with abandon. We'll capture Safari webarchives of corpus pages. We'll extract what we care about from them into a mock datastore off to the side so we can run our training and tests in node.