WebDeposit: sequential form rendering

• Implements a simple Article Workflow that renders two forms sequentially (ArticleForm, PhotoForm)

• Alters the WebDeposit tables.

• Adds functions to obtain a workflow get_latest_or_new_workflow, get_workflow and create_workflow.

• Adds workflow class functions to synchronize database and workflow object.

• When custom fields are imported from webdeposit_load_fields constructors now are attributes to an object called 'fields'.

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