HTML/CSS MVP - capability to replace trinket on web-intro path

[sra405]

Part of https://github.com/RaspberryPiFoundation/digital-editor/issues/94 but very much focused on the minimum requirements to get the basic functionality working and looking reasonably close to designs.

Closing out this Q2 OKR:

All projects in the HTML/CSS path can be migrated to the new Editor

design