Modify "cookie notice" banner on testserver

Related Issue

The cookie banner now displays a warning in demo mode that you should not enter sensitive data

[x] Is the code formatted correctly?
[x] Unnecessary comments removed?
[x] Printout statements removed?
[x] If you made backend changes: Did all tests pass? Do you need to adjust some tests or write new ones?
[x] If you made frontend changes: Did you test the UI on different devices/ browsers (Firefox, Chrome, Safari, different smartphone sizes)
[x] Is your code easy to understand or do you need to insert some comments?
[ ] Explain the issue for the reviewer and your steps to solve the issue => makes it a lot more easy for the reviewer
[ ] Label your pull request (frontend/ backend/ testing/ styling/ sql/...)
[x] Is the open source message included at the very beginning of every source code file?
[x] Are all imports organized, i.e. are the imports up to date and have all package imports been removed?