Don't propose unsigned transactions

[katspaugh]

In a 1/X Safe, it's possible to create and immediately execute a transaction w/o signing it.

We're currently "proposing" such txns to the backend, where they are marked as untrusted until indexed.

@Uxio0 says we shouldn't do this. The ability to "propose" w/o signatures will be removed in the future on the backend.

[iamacook]

We need to ensure this works well with smart contract wallet owners. Currently, if a Safe owns a Safe ("parent" vs. "nested"), the "parent" can queue the approval of a transaction of the "nested" Safe. As such, the "nested" Safe currently proposes a transaction without signatures.

[Uxio0]

I think we never tested EIP1271 signatures for multisig transactions, but they are implemented 😉. Also in that case a delegate could be used to make things easier