Rearrange and clean up more webhook stuff

src/client/Client.js

@@ -279,6 +279,15 @@ class Client extends EventEmitter {
     return this.rest.methods.getInvite(code);
   }
 
+  /**
+   * Fetch a webhook by ID.
+   * @param {string} id ID of the webhook
+   * @returns {Promise<Webhook>}
+   */
+  fetchWebhook(id) {
+    return this.rest.methods.getWebhook(id);
+  }
+
   /**
    * Sweeps all channels' messages and removes the ones older than the max message lifetime.
    * If the message has been edited, the time of the edit is used rather than the time of the original message.