Assign valuators when managing a single proposal in the admin

[microstudi]

Original requirement:

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

Context

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.

Tasks

• [x] Create in the admin, when managing and individual proposal a selector that allows the admin assign valuators directly in there:
• [x] Send a notification, same as #18 when a valuator is assigned.

Create the branch from release/0.26-stable, PR to this same branch in this repository

[paarals]

first assignt to valuator then, valuators users change mockup