Add proposals author in Notifications

Ref. PWA11.4

NOTE: this feature is from a big development. See PWA discussion at GitHub.

Is your feature request related to a problem? Please describe.

As a registered participant who is following content in Decidim, when there is a new proposal notification it doesn’t show the author. For instance, “The proposal Let admins edit in the backend meetings that they created in the frontend (and vice-versa) has been added to Propose new functionalities for Decidim software”

Describe the solution you’d like

I want to see the author. For instance, “The proposal Let admins edit in the backend meetings that they created in the frontend (and vice-versa) has been added to Propose new functionalities for Decidim software by Pierre Mesure @pierre”

Acceptance criteria

• Given that I’m a participant with a new proposal notification,
  When I go to my notifications page
  Then I can see the proposal author on the notifications page.