deposit: improvements for conferences field

depends on: https://github.com/inspirehep/inspire-next/pull/75, https://github.com/inspirehep/inspire-next/pull/125
Just wondering if we are using it right now. We just want to enable extensions that we really use so usually it would get enabled together with the commit that adds some tests. If there are some tests already in Invenio that we can run we can enable it.