Take db_column into account while syncing fields

Existing translated fields columns were not detected by `sync_translated_fields` command if the fields had a `db_column` attribute. 

And was thus failing as it was trying to create existing columns :

```
django.db.utils.ProgrammingError: column "nom_en" of relation "o_b_renseignement" already exists
```

I made a quick fix which works locally. With no unit tests so far... I would be pleased to improve this fix with your help :)