Experimental workflow management

[joemull]

This standardizes the behaviour of the workflow manager page we already have:

• Can move articles forward as well as back
• Logic allows movement to or from any stage recognized in the journal's workflow (via models.STAGES_ELEMENTS)
• Includes workflow plugins
• Does not depend on workflow logs as much, and creates them when needed as well
• Unassigned has a dedicated workflow element, instead of just being a stage with no element
• Only staff accounts can navigate to and use the tool, since it is easy to invalidate and mix up tasks if used without awareness of open tasks

Some stage-specific tasks and attributes are modified (the ones we already had support for) but otherwise moving an article does not change anything other than stage and workflow log.