sqlalchemy: add session_manager

* Adds a new utility decorator called session_manager for use
  in models to handle commit/rollback actions.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
workflows: new session_manager usage

* Removes duplicate session_manager and instead uses the one
  recently added in ext.sqlalchemy.utils.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
oaiharvester: add save to model

* Adds save method to OaiHARVEST model.

* PEP8/257 fixes.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
OAIHarvest: fix admin pages

* Fixes broken imports in admin pages of OAI harvest sources.
  (closes #2188)

* Bootstraps some templates for easier human interaction.

* Uses the oaiharvester.models.OaiHARVEST for interacting with DB.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>