Add notifications for trips and allow to mark trips as 'read'

[thisismeonmounteverest]

The trips feature at the moment isn't working as well as it could. One problem is that a lot of people do not use the landing page (as they seldom login) and another is that even if they do they always see the same trips as there is no way to remove trips of visitors.

Adding the following hopefully mitigates this:

• Send a notification when new trips are added in your area
  • Possible ways to do that: Immediately, once a day, once a week?
• Add way to hide a trip from the list of visitors, so that a new trip is shown instead (if any)

Notifications should only send as long as a member wants them, so an unsubscribe options is needed (can also be a preference). Unhiding a trip isn't needed but could be done on the general trips overview page.