Make global delegations explicit

[oliverbarnes]

Currently it's implicit that, if a proposal_url is nil when creating a delegation, it means it's a global.

(Soon a nil voting method will also be part of that)

To somebody new to the codebase, and even to us, this is hard to tell.

We should refactor to make global delegations a deliberate, clear thing from the highest level and down.

[oliverbarnes]

Maybe we should look into the api interface itself as well (in which case, this would be more than refactor)