(Feature request) force-add "unoffered this semester" class to road

[eiis1000]

Being notified that a class isn't offered in the semester you want to take it is great and the color-based indication is fantastic, but being entirely unable to add it is restrictive for planning purposes. It would be nice if it were still possible to add classes to semesters that they're not offered, possibly via an enable switch and/or warning dialog when you try to do so.

This request can also be alternatively resolved by merging #505, which would allow those planning issues to be resolved with custom classes.

[psvenk]

I second this, and I think it's a good quality-of-life improvement even if the critical need will be solved by #505.