base: pdf.js previewer

* Switches to new pdfjs previewer.

Reviewed-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>
base: integrate previewer module

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
installation: SQLALchemy 0.9+

* Removes locked down version of SQLAlchemy dependency after update in
  Invenio.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>