Group same-artefact environment review notifications in notifications page

If a user is assigned too many environment reviews the notifications page will be polluted with multiple links to the same page.

Notifications for reviewing environments of the same artefact should be grouped into an expansible list.

Acceptance criteria:
- The backend has a "batch dismiss" endpoint for notifications that allows dismissing a list of notifications at once
- Instead of showing each environment review notification, the frontend should show a single card with a button to expand and a "dismiss all" button
- The card should read "Environments for artefact X - version Y"
- Clicking the expand button should show the individual notifications
- When expanded, the "expand" button should become a "collapse" button.