Implement Schengen Area Compliance Report

How many close am I to 90 days in the last 180.

Fixes #193.

templates/navbar.html

@@ -14,6 +14,7 @@
   {"endpoint": "weekends", "label": "Weekends" },
   {"endpoint": "launch_list", "label": "Space launches" },
   {"endpoint": "holiday_list", "label": "Holidays" },
+  {"endpoint": "schengen_report", "label": "Schengen" },
   ] + ([{"endpoint": "birthday_list", "label": "Birthdays" }]
     if g.user.is_authenticated else [])
   %}