Proposal collection reference field

@oliverbarnes is already working on this.

Since Apr 29, 2021.

Decidim participatory budgets integration brings up the scenario of voting on a selection of projects:

> Open question: the delegation will be given within the scope of the budget - so neither globally, nor individually for each project/proposal. How do we solve that? We have the idea of delegations by category in the horizon, but I'm not sure that's a fitting abstraction for what is in the end a custom selection of projects. This scenario might come up again where there's multiple choices involved (the scope of a question, for instance, for multiple answers).
(https://github.com/liquidvotingio/decidim-module-liquidvoting/issues/45#issuecomment-819726486)

I'm thinking the API needs a proposal collection reference field for delegations that could account for different kinds of proposal buckets. This would still be external as the bucket will be platform-dependent. And the initial assumption, which we need to confirm, is that it'll have its own url.

Possible candidates:
- scope_url
- proposal_collection_url
- proposal_group_url
- proposal_collection_url

There's also the issue of whether `proposal` itself is still a valid abstraction (say people are voting on answers rather than proposals), that perhaps precedes this. I'll open a discussion for it.