Changed the lower bound for main post offices to 20 workers.

Fixed a bug in vehicle loads calculations.

Fixed another cause of notification spam.
Some code rearrangements.

Notifications now also trigger after clicking a building without access to mail.

1. Decreased mail production by 25%.
2. Increased the minimal time between notifications.
3. No postal service warning only triggers after changing the number of post offices to reduce notification spam.
4. Changed the way mail enroute is calculated.

Updated for 1.0.