Consolidates loading Braintree dependencies into a shared module instead
of copying it around each module that has a Braintree integration.

```js
import BraintreeLoader from 'path/to/./util/braintree-loader';

BraintreeLoader.loadModules('applePay', 'dataCollector')
  .catch(...)
  .then(...)
```

`ApplePaySession` is stubbed out for the unit tests, so it should not
matter which browser is running the tests.

chore: extract loading braintree libraries to util/braintree

To better align the Google Pay element with the Apple Pay element, this
refactors it to be a concrete class instead of a function. This will
help in the future when we need to specialize its functionality for a
different implementation of collecting the payment information. Adding
Braintree support is an example of that specialization.

chore: convert Google Pay element to a class from function

Ensures setup fees are not applied to subscriptions with quantity = 0

[Full Changelog](v4.29.2...v4.29.3)

**Merged Pull Requests**

- Ensures setup fees are not applied to subscriptions with quantity = 0 [#893](#893) ([chrissrogers](https://github.com/chrissrogers))
- chore: convert Google Pay element to a class from function [#892](#892) ([cbarton](https://github.com/cbarton))
- chore: extract loading braintree libraries to util/braintree [#891](#891) ([cbarton](https://github.com/cbarton))

##### Minified MD5 Checksum
```
3aedc5c6a5c87365133c8b23feb32f0c ./build/recurly.min.js
```
##### [SRI Hash](https://www.srihash.org/)
```
lhLNZJ447F9CH0rGPYMNS3NV3Nwnn4Y3oaUsbTa3cAQ/J7XDX76JAa2Htis2bEro
```

Ensures addons are not included in subscriptions with quantity = 0

[Full Changelog](v4.29.2...v4.29.4)

**Merged Pull Requests**

- Ensures addons are not included in subscriptions with quantity = 0 [#895](#895) ([chrissrogers](https://github.com/chrissrogers))
- Ensures setup fees are not applied to subscriptions with quantity = 0 [#893](#893) ([chrissrogers](https://github.com/chrissrogers))
- chore: convert Google Pay element to a class from function [#892](#892) ([cbarton](https://github.com/cbarton))
- chore: extract loading braintree libraries to util/braintree [#891](#891) ([cbarton](https://github.com/cbarton))

##### Minified MD5 Checksum
```
42050ba64172af4e291a37545fa905de ./build/recurly.min.js
```
##### [SRI Hash](https://www.srihash.org/)
```
OHnJ0LXZyg2EpXal64oDpbfmLPrToCaWpN792/a3zsj6my6ILT6bd9k7NVaZFxge
```