Hide hidden schemas

[nicklayb]

This PR hides fields relating to non allowed schemas in the Live View component.

Suppose you have a User that belongs to a Manager and have the following schemas

defmodule Definition do
  def schemas(%{role: "user"}), do: %{"User" => MyApp.User}
  def schemas(%{role: "admin"}), do: %{"User" => MyApp.User, "Manager" => MyApp.Manager}
end

Using assigns role: "user" shouldn't allow the UI to show the manager nor the manager_id keys on the User schema since they aren't allowed to use Manager schema.