Feat: add author notification on proposal publication

[Stef-Rousset]

🎩 What? Why?

The goal of this PR is to send a confirmation of publication, by notification and email, to the author of a proposal.

📌 Related Issues

• Related to https://meta.decidim.org/processes/roadmap/f/122/proposals/17894

Testing

1. After login, go to your account and then to notification settings, and check "Real time" for "How often do you want to receive the notifications email"
2. Go to a participatory process
3. Click on proposals
4. Add a proposal
5. Publish it
6. Verify that you receive a notification and an email

📋 Subtasks

• Add tests

[github-actions]

This pull request does not contain a valid label. Please add one of the following labels: ['type: feature', 'type: change', 'type: fix', 'type: removal', 'target: developer-experience', 'type: internal']

[BarbaraOliveira13]

I ran it and it works 🎉 LGTM 👍