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 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>
demosite: PendingDeprecationWarning on populate

* NOTE adds PendingDeprecationWarning for demosite populate command.
  (addresses #2394)

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