Create membership for user creating project <=> role exists
Fix validation of fields in partial updates

For some reason validation of fields in partial updates wasn't
triggered. Investigating I found that in the case of partial updates
every field is marked as not-required when constructing the serializer:

rest_framework/fields.py:173
        if self.partial:
            self.required = False

but I couldn't a find where the place (if such a place exists) where the
fields that came in the PATCH request are marked as required again in
order to trigger their validation.

Basically this hotfix does that, mark as required every field present in
the PATCH request so their validation is performed.