Fix groups mentions in Notifications

Ref. PWA11.5

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 you were mentioned as a group, there’s a bug that the notification doesn’t show which group you were mentioned. For instance, “You have been mentioned in Ability to disable public profiles by Carol Romero @carol as a member of”

Describe the solution you’d like

The notification should say which group was that: “You have been mentioned in Ability to disable public profiles by Carol Romero @carol as a member of Product Team @product”

Acceptance criteria

• Given that I’m a participant with a new user group mention notification,
  When I go to my notifications page
  Then I can see which group I was mentioned.