Iteration and wide refactor

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