Snaps: parameter rejig

New query params for draggable snap urls:

* `gu-snapUri` is as before
* `gu-snapType` now describes the Uri; valid values are `document`, `fragment`, `json.html`
* `gu-snapCss` now sets the `gu-snap--...` class

NB. `json.html` means a html property within json, like the current football embeds.

cc/ @adamnfish @jamesgorrie