Problems: Add Copy Code Button

[cuihtlauac]

It would be nice to have a button allowing to copy the problem code fragments, either in statement and solution.

[sabine]

What needs to be done:

1. extend problem.ml to provide statement_ml and solution_ml (or maybe there are better names) string fields that contain the strings that should be copied to clipboard. I think that means stripping the markdown code blocks and turning all non-code text into comments because it would be nice to have the full problem statement copied, not only the code block.
2. add a button in the UI to copy to clipboard.

[sabine]

After getting some feedback from the community it became clear that it is not that clear what should happen here: Some users would greatly prefer that exercises are also available for offline consumption, e.g. in a git repository or with a command line application that also checks that the exercise is solved correctly.

IMO it would be nice to have an "open in playground" button because the same code that is copied to the playground could also be made available separately.