Add example of association authorisation

[pmackay]

The configuration of assocation policies are somewhat tricky, it would be super helpful if this demo included methods in the policy classes to define association permissions.

[adrianthedev]

Hey @pmackay. Apologies for such a late reply. This issue slipped through the cracks in July.
Feel free to ping me again if I miss other issues like this one.

I pushed some code to show and example of how you'd show/hide buttons using the authorization feature for associations.

You can see the results on this team page when you scroll down to the Memberships section. You'll see that the create & attach buttons are missing, and the rows don't have any buttons on them.