records: fix iframe previewer and buttons

* Fixes iframe previewer for ispy-online. (closes #340)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>

Conflicts:
	invenio_opendata/base/templates/records/ALICE-Derived-Datasets_metadata.html
	invenio_opendata/base/templates/records/CMS-Derived-Datasets_metadata.html
	invenio_opendata/base/templates/records/files_base.html
	invenio_opendata/base/templates/records/macros.html
	invenio_opendata/base/templates/records/metadata_base.html