template: Flask 1.0 compatibility fix
template: Flask 1.0 compatibility fix

* Fixes return value of Jinja loader iteration due to change in
  Flask 1.0. (closes #2216)

Signed-off-by: Lars Holm Nielsen <7eaffca9f7cd98839b5821d0d23e09ff764b82cd@cern.ch>

inveniosoftware/invenio,inveniosoftware/invenio,tiborsimko/invenio,tiborsimko/invenio