#1083 SBE course improvements based on feedback

I've only addressed feedback from people that followed the course, as catalogued in #1083. This merge request does not aim to resolve all the other items on the list there.

End-user visible changes:

• Small improvements/fixes, including improvements/fixes, adding an extra screenshot, etc.
• Small quiz improvements/fixes, including splitting some questions into multiple questions to make it easier to do trial-and-error in case you really don't know the answer.
• Add extra links to more info about yEd and ToolDef.
• Updated expected course duration information, as it is clear that 20 hours is not sufficient. Updated to best new guesses, based on available information.
• I removed the 'under development' note on the course home page. So far, everybody that followed the course or a large part of it, has liked it a lot. They didn't comment on the order of things, that it was difficult to follow, or anything like that. Given that this was the biggest concern, it seems the course is already quite good, in practice. Sure, it could be better, as everything we have, and that probably holds even more so for this course, but in my opinion that no longer justifies the note. Even without the note, we can obviously still further improve the course.

Addresses #1083