Allow more guidance and / or checks for Workflow forks

[profcturner]

In some cases one workflow option is provided as a safety precaution. For instance, by default it is possible for a Module Coordinator to do final sign off without External Examiner sign-off. But this is intended only for this case.

Possible improvements:

• the AssessmentState object is augmented to have help for each state, and possible next states.
• the drop down options that aren't available should perhaps be there but greyed out, or otherwise shown somewhere?
• possibly allow another model that allows bespoke warnings from state A to state B.

[profcturner]

A next_states_guidance field, intended to be raw HTML has been added, and is displayed if existed.