WebStyle: use utf8 for jinja2 str to unicode

• Jinja2 requires all strings to be unicode. Invenio however operates with UTF8 encoded 8-bit strings. Jinja2 will automatically convert non-unicode objects into unicode objects, but using ASCII codec. This function replaces the escape function and Markup class in Jinja2, to use UTF8 codec instead when converting str objects into unicode objects.

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>