Wide refactor of style and hacks

Imported remaining TODO from #1:

 - [ ] weight / has_score / score event
 - [ ] saved, server side item state
 - [ ] server side checking and feedback
 - [ ] ability to edit in the builder
 - [ ] full HTML editor for the question text

Remaining remarks from @antoviaque:

 - [ ] HTML should be moved out of JS files, to proper templates.
 - [ ] missing tests
 - [ ] missing a README