2.x - Adds oauth authentication support along-side api key use

This branch adds functionality to replace the existing API Key authentication method with a new workflow that incorporates authentication through a Drupal Mailchimp middleware application.

Changes

• Replaces API key with Access token and Data center
• Adds new authentication page with
  • Form that POSTs to middleware server to initiate the authentication request
  • And triggers a new window for authenticating directly with Mailchimp
  • Simultaneously it starts pinging the middleware server for a status
  • When status returns accepted POSTs again to the server requesting the access token
  • Receives the access token and stores it along with the data center
• Updates the ClientFactory class to use access_token and data center with "OAuth" user
• NOTE: this update is also requires an updated version of the Mailchimp PHP Library (that is currently in a PR on that repo)

config/install/mailchimp.settings.yml

-api_key: ''
+api_key:
 api_timeout: 10
+domain: ''
+use_oauth: FALSE
+oauth_middleware_url: 'https://drupal-mailchimp-oauth.uw.r.appspot.com'
 cron: FALSE
 batch_limit: 100
 api_classname: 'Mailchimp\Mailchimp'