When editing Nspaces, although you can assign policies to a nspace using
PolicyDefaults you cannot assign a Service Identity to a policy like you
can when adding a policy to a token.
This commit adds an extra attribute to our policy-form/policy-selector
component so you can disable this setting. At a later date we may change
this to have a conficgurable `<Slot />` instead.
Simple acceptance tests is included here
John Cowen