add endpoint for calibration level

[jeffeaton]

Currently spectrum calibration level gives options "None", "National", "Subnational" for all cases. Ideally this should only show "None" or "National" if it's a national Spectrum file. This endpoint can be delivered based on if the all(spectrum_region_code == 0) in the area hierarchy.

This applies to the Population calibration on the Model options page, and all four calibration options on the Model calibration page.

[r-ash]

To get this working on the calibration options page requires a bit of a change as currently the endpoint takes no inputs. We would have to update the endpoint to take PJNZ file or areas file to be able to know if national/subnational Spectrum file.