Deposit: user side deposition ending without workflow finishing

* finalize_record_sip task seals the sip before updating
  the object.

* DepositionType class inherits from WorkflowBase in order to
  be displayed in holdingpen.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
workflows: signals added
workflows: PEP8 fixes