global: url_for unicode compatibility fix
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 <5460ec99b71a72e7693c4088b5c2585e76e1c857@cern.ch>
Reviewed-by: Jiri Kuncar <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>

inveniosoftware/invenio-base,tiborsimko/invenio-base,tiborsimko/invenio-base,inveniosoftware/invenio-base