Enable automatic model runs

Button on admin page:
1. saves the 'automatic model runs start date' (changed from boolean),
2. sets the 'class changed date' on all 'admin unit disease extent classes' for the disease group,
3. adds validation parameters for all of its occurrences (yet to have final weightings assigned).

Can only be pressed if 'model run information' response says the model can be run, (ie all required parameters are provided and valid).