remove inline js for favicon

[mawinter69]

favicon generates inline js which is not CSP compliant. This changes it so the url is provided via a data-url attribute of the script.
Also simplifies a bit the javascript and uses modern syntax.

Testing done

Manual testing that favicon still works
Verified with CSP plugin that there are no errors related to favicon

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[TobiX]

Thanks!