Astonishingly, the great jinja2 does not provide a builtin urlquote filter,
although it is obviously needed. (jina1 had one) This is:
Provide an urlencode filter, based on werkzeug's url_quote_plus function.
This is dead easy to implement and gives us all the freedom we want.
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>