timeline: Added new "Timeline" view

Similar to the "Notifications" view, but for the whole application.
notifications: Removed flight filter option

Has that ever been used?!
notifications: Extracted GROUPABLE_EVENT_TYPES list constant
notifications: Group follower notifications too
notifications: Use "You" if event.actor == current_user
notifications: Renamed notifications-table template to events-table
events: Adjusted FOLLOWER event texts
notifications: Added documentation
notifications: Moved EventGroup/group_events() to model
model/notification: Renamed module to `event`
timeline: Added "Timeline" button to user profiles
notifications: Improved _filter_query() function

Make sure we don't filter for non-integer values
timeline: Show username in title if requested
events: Added NEW_USER event type
i18n: Updated translation template
i18n: Updated German translation
Merge pull request #117 from Turbo87/timeline

Added "Timeline" views