[5.3] Add isRead and unread method to DatabaseNotification (#17243)

* add isRead and isNotRead to DatabaseNotification

* change isNotRead to notRead

* finally change notRead to unread

* Update DatabaseNotification.php

src/Illuminate/Notifications/DatabaseNotification.php

@@ -57,6 +57,26 @@ public function markAsRead()
         }
     }
 
+    /**
+     * Determine if a notification has been read.
+     *
+     * @return bool
+     */
+    public function read()
+    {
+        return $this->read_at !== null;
+    }
+
+    /**
+     * Determine if a notification has not been read.
+     *
+     * @return bool
+     */
+    public function unread()
+    {
+        return $this->read_at === null;
+    }
+
     /**
      * Create a new database notification collection instance.
      *