Make separation between user-access and admin-access endpoints

[pedropregueiro]

We need to have a way of showing people the different access level needed when using specific API endpoints. For example, to create a user POST /v2/users or a subscription POST /subscriptions you can do it using your normal user JWT. However, to create a plan POST /plans or to create a product POST /products you need to be an admin, therefore to use an admin JWT.

We can do this by using either labels (admin, user) on the endpoint or just add a label admin to the admin specific ones. Need to think about the options