Currently we use the unique_together constraint to ensure that if a label has a
shortcut key combination, no other label has the same combination.
However, on SQL Server, the unique_together constraint also looks at null
values which breaks the unique_together constraint since we now can no longer
have two labels without shortcut keys. The error message is as follows:
```
Violation of UNIQUE KEY constraint 'api_label_project_id_prefix_key_suffix_key_1b3d8f77_uniq'.
Cannot insert duplicate key in object 'dbo.api_label'.
The duplicate key value is (32, <NULL>, <NULL>).
```
Moving the validation logic from the database into Django fixes the issue and
makes the constraint clearer.
Clemens Wolff