Viewer export options

[slimbuck]

This PR:

• adds a popup dialog for viewer export options
• populates the settings with editor settings the first time it's shown
• adds support for generating a .zip package instead of single-file html
• shows a filename field when running in safari (which doesn't support file picker)

Notes:

• I haven't managed to include JSZip correctly the usual way, so it's being raw-dog loaded in the application's index.html
• the structure of settings etc is messy and needs a cleanup
• eventually the new viewer export dialog should form the basis of a reusable settings dialog, including scss

[willeastcott]

Can't wait to play with this!! 👀