R09. Assign valuators directly in the individual proposal

[paarals]

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.

Mock up

Does this issue could impact on users private data?

Acceptance criteria

• Assign valuators directly in the individual proposal

Technical specifications
openpoke/decidim#20

Implementation

Testing

Additional context
Metadecidim proposal: https://meta.decidim.org/processes/roadmap/f/122/proposals/17114

[Naadjjaa]

@paarals Perfect!