template: 'u' filter addition

• NEW Adds 'u' filter that converts str to unicode in Jinja2 templates since support for str has been deprecated. Example: {{ mystr|u }}. (addresses #2862)

Co-authored-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>