New task form: need to reset VDAF parameters when changing VDAF type

When the vdaf.type field is changed in the new task form, we need to both clear any nested parameters that don't apply to the new VDAF, and set default values of VDAF parameters to mirror the default values of their controls. Currently, if you change the "Function" selector to "sum" and don't touch the "Measurement Range" selector, the request will only include a type within the vdaf nested object. Moreover, the API error (below) is not surfaced as a validation error in the UI.

{
	"vdaf": {
		"bits": [
			{
				"code": "required",
				"message": null,
				"params": {
					"value": null
				}
			}
		]
	}
}

Furthermore, if you change the function to "histogram", enter some histogram buckets, and then change the function to "sum", the histogram buckets will still be sent to the server in the request.

Start by locating the new task form and the handler for changes to the vdaf.type field. Trace how control defaults and nested VDAF values are assembled into the request, then check how the shown API validation response is mapped into UI errors. Done means irrelevant parameters are cleared, applicable defaults are submitted, and VDAF validation errors appear in the form.