Insert new base exception for OpenSlides.
Work on motion workflow system.

Also: Insert new base exception for OpenSlides.

Also: Insert a workflow field to the state class. Update tests. Rename versioning attribute.
Merge branch 'OpenSlidesError' into workflow_2

Conflicts:
	openslides/utils/exceptions.py
Comment the dev-file workflow.py
Move motion exceptions to its own file in the motion app.
Fix PEP8 and tests.
Merge pull request #531 from normanjaeckel/workflow_2

Work on workflow system. Now manager can choose the default workflow for a motion.