implement the persistence of data in DB module.

The puzzle `102-bc876a9d` from #102 has to be resolved:

https://github.com/ammaratef45/Attendance/blob/fbdba811a613eddf24735e47ac232e93e71c179b/attendance/lib/backend/attendance.dart#L62-L62

The puzzle was created by Ammar Atef on 12-May-19.

If you have any technical questions, don't ask me, submit new tickets instead. The task will be \"done\" when the problem is fixed and the text of the puzzle is _removed_ from the source code. Here is more about [PDD](http://www.yegor256.com/2009/03/04/pdd.html) and [about me](http://www.yegor256.com/2017/04/05/pdd-in-action.html).