Generate Disease Extent button (system administration for diseases)

* Add a Generate Disease Extent button to the Model Runs section of system administration for diseases
* The button is only enabled if the model has yet to be run, allowing the disease extent to "look good" before the initial model run is done
* Disease extent generation always uses all disease occurrences if the model has yet to be run

wording: "enables IF the disease extent is able to be generated"