Moving group and event notifications to the Activity section

Notifications of new posts in groups and events you belong to have been moved from the Notifications section to the Activity section. The change aims to improve logic and reduce the number of notifications:
- Notifications only display events relating to your own profile, such as receiving a like, comment or invitation, and those relating to groups/events you own, such as new volunteer applications.
- Activity displays events relating to the profiles of friends/following and groups/events you belong to, such as a friend adding a photo or a new post in a group.