sqlalchemy: custom EncryptedType removal

* Removes the custom EncryptedType and uses implementation from
  SQLAlchemy-Utils.  (closes #2343)

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
previewer: pdfjs plugin addition

* Adds pdfjs previewer.

Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>