Move first_content_section_id to workSchema.js.

Because it has to do with the models, I guess? I dunno. Also not a huge fan
of this being a global function - is there a better place to put it?
Change first_content_section_id to iterate_content_ids, for more general content id iteration.

We find the first by simply returning the first one we find.
Don't expect to be in the context of a section object in the "content" template.
Add a "next section" link at the bottom of the section text.