Create test helpers for policies to test for actions and actions on objects

It would be really helpful to have a quick way to test that a policy implements given actions or actions on objects. Currently we have to do something like:

```
    it "has the expected actions" do
      user = build(:user)
      policy = AccessControl::RolePolicy.new(user)
      expect(policy).to respond_to(:create_edit_roles?, :manage?)
    end
```