Changes at "Assign valuators when managing a single proposal in the admin"

Is your feature request related to a problem? Please describe.
Currently valuators can be assigned in bulk by clicking a checkbox in the proposal's index of the admin (see #18)

When answering/managing a single proposal from the admin list it is not possible to assign a valuator there.

Describe the solution you'd like
As a process-admin, I would like to be able to assign valuators directly in the individual proposal.

Describe alternatives you've considered

Welcome!

Additional context

Mock up

Technical specifications: https://github.com/openpoke/decidim/issues/20

Related to: gid://metadecidim/Decidim::Proposals::Proposal/17023

Does this issue could impact on users private data?

No

Funded by

City of Luzern

*We are aware that there is a redesign policy (https://github.com/decidim/decidim/discussions/9512), but we would like to know if this can be accepted once the redesign is do