general: importing modules from packages fix

* Uses Flask-Registry for importing modules from packages.

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
sqlalchemy: custom encrypted type removal

* Removes the custom Encrypted type that was implemented
  for invenio, since SQLAlchemy-Utils comes with an
  EncryptedType implementation. (closes #2343)

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>