Remove {slug} from contracts

[olf42]

Contracts have a unique identifier (cid) and can be identified better by giving the name or email address of the other party. Therefore I propose to remove the slug property from the contracts.

[olf42]

Furthermore: If no slug is set, print-contracts will print unknown. If this is a property used to distiguish contracts in this view, a different property should be used.

[aparcar]

I like the idea of having a slug or nick which is easier to remember than a long number...

[olf42]

Good point.
How do we ensure that it's unique?