feat(Amplitude): enable referrer

react/features/analytics/handlers/AmplitudeHandler.js

             host
         };
 
-        amplitude.getInstance(this._amplitudeOptions).init(amplitudeAPPKey);
+        amplitude.getInstance(this._amplitudeOptions).init(amplitudeAPPKey, undefined, { includeReferrer: true });
 
         if (user) {
             amplitude.getInstance(this._amplitudeOptions).setUserId(user);

react/features/analytics/handlers/amplitude/Amplitude.native.js

      * Sets an identifier for the current user.
      *
      * @param {string} userId - The new user id.
+     * @param {string} opt_userId - Currently not used.
+     * @param {Object} opt_config - Currently not used.
+     * @param {Function} opt_callback - Currently not used.
      * @returns {void}
      */
-    setUserId(userId) {
+    setUserId(userId, opt_userId, opt_config, opt_callback) { // eslint-disable-line camelcase, no-unused-vars
         AmplitudeNative.setUserId(this._instanceName, userId);
     }