Update Braintree SDK to v3.0

[toomuchpete]

Our current payment system uses Braintree's Javascript SDK v2.0. We should upgrade to v3.0.

Payment front-end can be found in the Registration Controller's Pay view, while processing code (not likely to be needed for this) can be found in the Payments Controller.

To get this working in a local environment, you'll need a Braintree sandbox account. Those keys can be placed in the .env file, like so:

braintree_environment: development
braintree_merchant_id: MERCHANT_ID_HERE
braintree_public_key: PUB_KEY_HERE
braintree_private_key: PRIVATE_KEY_HERE
braintree_cse_key: HUGE_LONG_CSE_KEY_HERE

Completing this ticket will likely make #122 easier.

[toomuchpete]

Moved to trello: https://trello.com/b/eQmfnejB