flights/view: Show overview and comments as tabs
flights/view: Moved basic statistics into separate tab
model/notification: Added user column to events table
notifications-table: Implemented "A is following B now." events

These are no longer restricted to show "A is following you now."
views: Mark flight/user notifications as read if related page is requested
views/notifications: Inject count_unread_notifications() into template

Defer the notification counting until the template is processed to
allow notification update processes to run before. This way if you visit
a user/flight page any notifications for that page will be marked as read
*before* the unread notifications are counted.
notifications-table: Link usernames to their profiles
notifications-table: Show subevents as smaller listing
notifications-table: Show small unread badges for subevents too
notifications-table: Translate "Show" button caption
notifications-table: Adjusted styling
views/flights: Mark flight notifications read if pilot flights requested
notifications-table: Simplified render_event_icon() call
views/notifications: Renamed events attribute to subevents
notifications: Removed link attributes

Generate the proper link in the template depending on the event type
notifications: Allow flight comment notifications on any flight
notifications: Added `grouped` query parameter
notifications: Moved CSS into table-events class
notifications: Added links for subevents
notifications: Adjusted subevent styling