Ensure airtable.js browser build works

- Regenerate the the airtable.browser.js via grunt and ensure that it
works via the python server
- Replace `node-fetch` and `abort-controller` packages with the build in
versions on the window for the browser build
- Maybe they should instead be replace by polyfills. There is a lack of
clarity of what browsers needs to be supported.
- Both fetch and abortController are widely supported by many browsers
so I think the polyfills can be skipped.

https://developer.mozilla.org/en-US/docs/Web/API/AbortController#Browser_compatibility
https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API#Browser_compatibility

lib/abort-controller.js

@@ -0,0 +1,3 @@
+var AbortController = require('abort-controller');
+
+module.exports = typeof window === 'undefined' ? AbortController : window.AbortController;

lib/base.js

@@ -4,8 +4,8 @@ var forEach = require('lodash/forEach');
 var get = require('lodash/get');
 var assign = require('lodash/assign');
 var isPlainObject = require('lodash/isPlainObject');
-var fetch = require('node-fetch');
-var AbortController = require('abort-controller');
+var fetch = require('./fetch');
+var AbortController = require('./abort-controller');
 
 var objectToQueryParamString = require('./object_to_query_param_string');
 var AirtableError = require('./airtable_error');

lib/fetch.js

@@ -0,0 +1,3 @@
+var fetch = require('node-fetch');
+
+module.exports = typeof window === 'undefined' ? fetch : window.fetch;

lib/run_action.js

@@ -3,9 +3,8 @@
 var exponentialBackoffWithJitter = require('./exponential_backoff_with_jitter');
 var objectToQueryParamString = require('./object_to_query_param_string');
 var packageVersion = require('./package_version');
-
-var fetch = require('node-fetch');
-var AbortController = require('abort-controller');
+var fetch = require('./fetch');
+var AbortController = require('./abort-controller');
 
 var userAgent = 'Airtable.js/' + packageVersion;