views/notifications: Removed joined loading of flight_comment details

We don't show the contents of the comment in the notification so we don't
need to load the comments.
model/notification: Renamed user parameters to recipient

Matches the column name
scripts: Added notifications/mark_all_unread.py
views/notifications: Focus on events, not notifications
views/notifications: Renamed id to notification_id
views/notifications: Added EventGroup tuple
views/notifications: Renamed notification to event where appropriate
views/notifications: Generate links in view
views/notifications: Use imap() instead of map()
views/notifications: Simplified code for 'type' in request.args
views/notifications: Extracted _group_events() function
views/notifications: Reduce excessive grouping

Only group flight notifications when there are no other notifications
in between them.
views/notifications: Renamed `sender` parameter to `user`
views/notifications: Converted EventGroup tuple to class
templates/notifications: Increased icon size
notifications: Added "New" badges for unread notifications

Since we only show unread notifications at the moment this will be
displayed for all of them... WIP
notifications: Show already seen notifications too
notifications: Removed params parameter from render_template call
skylines: Added configure_jinja() method
notifications: Added pagination