Add backend config for site ID

Users can now specify `site_domain` in `config.yml`. This is useful
when attempting to access the CMS from a different host.

src/backends/github/AuthenticationPage.js

@@ -17,7 +17,7 @@ export default class AuthenticationPage extends React.Component {
     if (document.location.host.split(':')[0] === 'localhost') {
       auth = new Authenticator({ site_id: 'cms.netlify.com' });
     } else {
-      auth = new Authenticator();
+      auth = new Authenticator({ site_id: this.props.siteId });
     }
 
     auth.authenticate({ provider: 'github', scope: 'repo' }, (err, data) => {

src/containers/App.js

@@ -51,6 +51,7 @@ class App extends React.Component {
     runCommand: PropTypes.func.isRequired,
     isFetching: PropTypes.bool.isRequired,
     publishMode: PropTypes.oneOf([SIMPLE, EDITORIAL_WORKFLOW]),
+    siteId: PropTypes.string,
   };
 
   static configError(config) {
@@ -87,6 +88,7 @@ class App extends React.Component {
             onLogin: this.handleLogin.bind(this),
             error: auth && auth.get('error'),
             isFetching: auth && auth.get('isFetching'),
+            siteId: this.props.config.getIn(["backend", "site_domain"]),
           })
         }
       </div>