Allow mention notifications to override a user's notification settings

Desired solution: - A setting on the employee's own profile that controls whether being tagged always triggers a notification - When enabled, a tag notification is delivered even if the employee has their other notifications switched off - The employee can turn this behaviour on or off themselves Current behavior: - Turning off notifications suppresses all notifications, including direct mentions - There is no way to make an exception for being tagged Use case: Management who turns off notifications to avoid being interrupted by general timeline activity, but who still wants to be alerted when a colleague tags them directly in a message or comment.