Allow disabling of user namespace onboarding through configuration

You should be able to choose whether or not you want to let user create namespaces for themselves.

This can be done by disabling "user" endpoints on the API through some property :
- e.g. `io.insee.dev.k8sonboarding.user-namespace-enabled: true`

And on the UI, by disabling access to components using the API with an env variable :
- `USER_ENABLED=true`