[#73958524] Create dashboard form

I would appreciate feedback on:

1. How to make the tests less verbose
2. Our discrepancy between the `dashboard` and `dashboard-editor` permission
3. Anything else that is broken

On the verbosity thing, I reckon the tests are pretty reasonable from my initial scanning. Could you use this though?:

https://github.com/alphagov/performanceplatform-admin/blob/master/tests/admin/support/flask_app_test_case.py#L62