Generalize survey question creation

This abstracts the process of creating survey questions. Instead of directly writing HTML, you make a Question (either a Fixed, Scale, or Essay Question) and pass in a bunch of parameters. The survey-gen.js classes then do all of the work of setting up DOM nodes.

This implements a basic Fixed question. Scale and Essay questions are still TODO, but the groundwork is here.
No, it won't be localized or translated. I also want to make it really easy to quickly edit these at a glance, so that other people (non-SWEs) can add surveys without going through me. So I'd prefer to not have to add another layer of indirection.