Fix W340 warnings
Fix W340 warnings

https://docs.djangoproject.com/en/1.11/ref/checks/

fields.W340: null has no effect on ManyToManyField.

FOSDEM/volunteers,jrial/fosdem-volunteers,FOSDEM/volunteers,FOSDEM/volunteers,jrial/fosdem-volunteers,jrial/fosdem-volunteers,jrial/fosdem-volunteers,FOSDEM/volunteers