global: url_for unicode compatibility fix

• INCOMPATIBLE Changes url_for behaviour to return always a unicode string. (addresses #2967)

• INCOMPATIBLE Deprecates invenio.config hack for legacy code. (addresses #3106)

Signed-off-by: Leonardo Rossi <leonardo.r@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>