Add notification to valuators when a proposal is assigned

[mllocs]

:tophat: What? Why?

Sends a notification to the valuator when a proposal is assigned to them.

:pushpin: Related Issues

Closes https://github.com/decidim/decidim/issues/12909

Testing

• Assign a proposal to a valuator, the valuator receives a notification and an email. https://decidim-lot2.populate.tools/admin/participatory_processes/branch-highway/components/4/manage/
• The valuator can opt out from receiving a notification when a proposal is assigned to them. https://decidim-lot2.populate.tools/notifications_settings

:camera: Screenshots

:hearts: Thank you!