Fix QR tool and update model-viewer HTML

[Carifio24]

This PR makes the following changes:

• Fix the QR tool, which was broken after #35
• Update the styling of the model-viewer HTML - in particular, the "View in AR" button is now much more prominent
  • Additionally, we now store the HTML and CSS templates for the exported model-viewer page in resource files, and combine these (with appropriate substitutions) to generate the output HTML. For convenience, this styling information is still included as a style tag in the HTML (that is, the export is one file). I suppose we could make this an option if we wanted to?

[Carifio24]

codecov is failing but our test suite is pretty minimal at the moment so I'm not concerned with that