Extend CI check for correct file headers to JS files.

[terhorstd]

Describe the bug
The check_copyright_headers.py will notify if code files are missing the usual header. This currently does not include *.js files used in the documentation.

To Reproduce
CI passes, even though doc/htmldoc/static/js/filter_models.js is missing a header.

Expected behavior
Same as for Python and C++ files also the JavaScript and CSS components should have a proper header.

[github-actions]

Issue automatically marked stale!