Frontend improvements

Description / Overview

Student solutions quickly "get dirty" from correction comments scattered in. Further, in case of long sample solutions, scrolling down the sample brings the student submission out of view.

Use cases

quality support for the correction process

Links / references

Feature checklist

• a button that pops up a window with the clean view (= solution w/o comments) (could perhaps be implemented by "misusing" the copy-to-clipboard-button?)
• [ ] independently scrollable parts of window for submission and sample solution bar (already mentioned in #101 (closed))

(Make sure these are completed before closing the issue, with a link to the relevant commit.)

Feature proposal