demosite: update demosite package for create/populate

* Changes from invenio_demosite to invenio_demosite.base

Signed-off-by: Javier Martin Montull <javier.martin.montull@cern.ch>
installation: MANIFEST template fix

* Fixes the MANIFEST template to include all jsonext configuration
  files.

Co-authored-by: Esteban J. G. Gabancho <egabancho@gmail.com>
Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>