Closes #20301: Add "Clear all" action to notifications dropdown

[pheus]

Fixes: #20301

Introduce a GET-based view that allows a user to clear all unread notifications with a single action from the bell dropdown. The dropdown template is updated to show a “Clear all” control only when unread items exist. This streamlines notification management while preserving existing behavior for dismissing a single notification.

Summary of changes

• New view: NotificationDismissAllView registered via @register_model_view(Notification, name="dismiss_all", path="dismiss-all", detail=False).

  • Behavior: Deletes only unread notifications for request.user.
  • Response:
    • HTMX requests return the existing htmx/notifications.html partial (so the dropdown re-renders in place).
    • Non-HTMX requests redirect to account:notifications.

• Template update (dropdown):

  • Uses hx-get to call extras:notification_dismiss_all, hx-target="closest .notifications", and an i18n’d confirm prompt: “Clear all unread notifications?”
  • The action is hidden when there are no unread notifications.

• No changes to models, REST API, or existing single-item dismiss behavior.

Screenshots

Screenshots (notifications present)

Screenshots (no notifications)

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Notes for reviewers

• Happy to adjust copy (“Clear all” vs. “Clear all unread”), icon placement, or spacing if you have a preferred convention.
• Let me know if you’d like this action to target all notifications (read + unread); currently it’s scoped to unread for parity with the dropdown contents.