Add imprint URL to Template.pageFooter #16966

Description:

This pull request is a follow up on PR #13219 and adds an imprint URL to the page footer.

Fixes: #16965

Review

• [ ] Functional review done
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done see checklist
• [ ] Code review done
• [ ] Tests were added if useful/possible - but not tested, see above
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed

My first idea was to switch to a multituple (label and URL) to handle future requests for more URLs, too, but in the end I went with this simple solution as I couldn't deduct a way to handle label translations from the existing code I read.