Use sql query in User.get_follow_requests/1 for filtering logic

get_follow_requests can become slower as the amount of followers grows. Debugging showed that get_follow_requests fetches all user follow activities and filters them in app memory. Follow activities are created on every follow request (regardless of whether the user is locked or not, and their state isn't changed from 'pending' once the follow request is accepted), so for a user with N followers, it has to filter through N follow activities.