next/store: "course" purchases

[haraldschilly]

Adds a "Course" section to the store, to simplify purchasing a license for a course with a given number of students. It also adds a HelpIcon tooltip help for idle timeout – and makes that component work for next.js.

The main problems I faced in this PR is how URL vs. form values are handled. I don't know why this is so tricky. Probably something is off how the form is used. In any case, I added explicit default parameters in the initialization, such that everything is in the range of allowed combinations. This is important, because components in the form trigger additional processing of the URL. That's why there is now a flag for the actual initialization and also a flag for parsing the URL.

What I tested is tweaking the form and menu navigating. Then of course, saving the configuration to the cart. Then editing the license, having more than one license in the cart, etc. ... Purchase just works as before, the only new identifying field is source. No surprise, because after all, this is still the same type of license.

Also, as discussed, this is also recorded in the "info" field of the license, i.e.

# select * from site_licenses order by created desc limit 1;
info         | {"purchased": {"type": "quota", "user": "academic", "source": "course", ...}}

store index page

course page

idle timeout help popup

this is also on other pages

consistency check: "course/standard" vs. "site license / standard"

academic, whole of august, 61 students