Miniscule updates, reminder implementation

 - Added javadocs for `Notification`
 - Made field `plugin` `final` in `CommandsGeneral`
 - Made listeners and executors and backends `final` in main class
 - Renamed `getSortingCommands()` to `getSortingCommandExecutor` for clarity
 - Created class `CommandsNotes` for reminders, notes, and mail
 - `NotificaitonsBackend` work
 - - Migrated code from `GetNotifications()` to a method `NotificationFromFile()`
 - - Added `GetNotification()`
 - Created and implemented `/remind` and `/del-reminders`