Display Braintree subscription information for users

[jgdreyes]

Display current subscription information on memberships#show.

• Retrieve information from braintree
• To get all subscriptions, get customer from braintree and collect all subscription objects on each of the customer's payment objects.

# Something like this
customer.payment_methods.flat_map(&:subscriptions)

I'm not sure if we should do this or if we should only get the information for the subscription we know about. Let's chat about this if you pick this up @cbdileo.