Add support for closed book testing

[jorg-vr]

In collaboration with safe exam browser this would require of us to provide a 'link' to a series which does allow you to access the exercises within that series but doesn't provide any links to other dodona pages.

See discussion https://github.com/orgs/dodona-edu/discussions/6001#discussioncomment-11585957

[jorg-vr]

Another related issue has come up: some teachers want to allow only access to dodona using whitelisting of IP's

Supporting this would require two additional fixes:

• Have a fixed ip available for exam modes
• Make sure all important code is hosted by us instead of using CDN's

[pdawyndt]

Another related issue has come up: some teachers want to allow only access to dodona using whitelisting of IP's. Supporting this would require two additional fixes.

I guess we also have to warn teacher about linking external content (images, videos, data files, ...) into exercise descriptions. They should be recommended to host such content directly in the media folder of the exercise (or the public folder of the repo). Same for style sheets or JavaScript libraries used in descriptions.