Add `unique_together` constraint on `djwebdapp.models.Blockchain`
djwebdapp.models.Account
should have a unique_together
constraint on ("blockchain", "address")
since this constraint is necessary in the Tezos provider: https://yourlabs.io/pyratzlabs/djwebdapp/-/blob/master/src/djwebdapp_tezos/provider.py#L109
Indeed, this line will fail if there are multiple accounts with the same address/blockchain
pair, we should thus constraint it at the model level.