Deleting a note should really delete the note in database

Hi,

Currently, it seems like deleting a note just marks the note as deleted (by adding the deletion date in the deletedAt field), but keep the note in the database.

Since all notes are stored in plaintext, I know that the database administrator is able to see all the active notes, so user privacy is limited, but I think that a deleted note shouldn't be available, like, ever.

Demonstration :

```
mysql> select * from Notes where content like '%mysecretnote%';
+--------------------------------------+-----------+-------+------------+-----------+----------+----------------------------------------+-----------------------------------------------------------------------------+---------------------+---------------------+---------------------+---------------------+-----------+--------------------------------------+--------------------------------------+
| id | shortid | alias | permission | viewcount | title | content | authorship | lastchangeAt | savedAt | createdAt | updatedAt | deletedAt | ownerId | lastchangeuserId |
+--------------------------------------+-----------+-------+------------+-----------+----------+----------------------------------------+-----------------------------------------------------------------------------+---------------------+---------------------+---------------------+---------------------+-----------+--------------------------------------+--------------------------------------+
| 8b530b18-0796-4833-a213-81189cadef20 | SJR4bZuKz | NULL | private | 0 | Untitled | mysecretnote that I don't want anymore | [["8c158b3e-ce0b-4550-8347-4f533f50aaba",0,38,1521123644145,1521123662902]] | 2018-03-15 14:21:03 | 2018-03-15 14:20:38 | 2018-03-15 14:20:38 | 2018-03-15 14:21:03 | NULL | 8c158b3e-ce0b-4550-8347-4f533f50aaba | 8c158b3e-ce0b-4550-8347-4f533f50aaba |
+--------------------------------------+-----------+-------+------------+-----------+----------+----------------------------------------+-----------------------------------------------------------------------------+---------------------+---------------------+---------------------+---------------------+-----------+--------------------------------------+--------------------------------------+
1 row in set (0.00 sec)
```

After deletion :
```
mysql> select * from Notes where content like '%mysecretnote%';
+--------------------------------------+-----------+-------+------------+-----------+----------+----------------------------------------+-----------------------------------------------------------------------------+---------------------+---------------------+---------------------+---------------------+---------------------+--------------------------------------+--------------------------------------+
| id | shortid | alias | permission | viewcount | title | content | authorship | lastchangeAt | savedAt | createdAt | updatedAt | deletedAt | ownerId | lastchangeuserId |
+--------------------------------------+-----------+-------+------------+-----------+----------+----------------------------------------+-----------------------------------------------------------------------------+---------------------+---------------------+---------------------+---------------------+---------------------+--------------------------------------+--------------------------------------+
| 8b530b18-0796-4833-a213-81189cadef20 | SJR4bZuKz | NULL | private | 0 | Untitled | mysecretnote that I don't want anymore | [["8c158b3e-ce0b-4550-8347-4f533f50aaba",0,38,1521123644145,1521123662902]] | 2018-03-15 14:21:03 | 2018-03-15 14:20:38 | 2018-03-15 14:20:38 | 2018-03-15 14:21:03 | 2018-03-15 14:21:18 | 8c158b3e-ce0b-4550-8347-4f533f50aaba | 8c158b3e-ce0b-4550-8347-4f533f50aaba |
+--------------------------------------+-----------+-------+------------+-----------+----------+----------------------------------------+-----------------------------------------------------------------------------+---------------------+---------------------+---------------------+---------------------+---------------------+--------------------------------------+--------------------------------------+
1 row in set (0.01 sec)
```

Thanks for your work :)