bda.views.etat_places
- Use select_related on spectacles_set, avoid query issue for each spectacle.
- `slots__sum` is computed in view, instead of a query.
- Cleaner code (and avoid useless computations).
bda.views.places
- Add select_related on places, avoid query issue for each spectacle.
bda.views.inscription
- 1 query for spectacle field choices, instead of (#forms in formset *
#spectacles)
- Delete BaseBdaFormSet. The validation was redundant with
`unique_together` of ChoixSpectacle model.
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