Add more DB/model check on "Contract" entity

[Atala]

It seems to me that the the Contract entity :

• is linked one to one either to a business restaurant group or to a restaurant
• not both business restaurant group and restaurant at the same time

actually it seems to me that it is just an intermediary table to link PricingRuleSet with other items to apply it

So we should

• make the linkS between Contract and other entities one to one
• add a check that it is link to exactly one entity

I dont know how to do it in doctrine, but that is how to do it in postgres directly CHECK (email is not null OR phone is not null) (ref https://www.postgresql.org/docs/current/ddl-constraints.html )