Add trips page

Creating a new entity called a trip. This will group together any travel
accommodation and conferences that happen together on one trip.

A trip is assumed to start when leaving home and finish when returning
home.

The start date of a trip in is the trip ID. The date is written in ISO
format.

This assumes there cannot be multiple trips one one day. This assumption
might be wrong, for example a morning day trip by rail, then another
trip starts in the afternoon. I can change my choice of using dates as
trip IDs if that happens.

Sometimes during the planning of a trip the start date is unknown. For
now we make up a start date, we can always change it later. If we use
the start date in URLs then the URLs will change. Might need to keep a
file of redirects, or could think of a different style of identifier.

Trip ID have been added to accommodation, conferences, trains and
flights.

Later there will be a trips.yaml with notes about each trip.

templates/navbar.html

@@ -2,6 +2,7 @@
 
 {% set pages = [
   {"endpoint": "index", "label": "Home" },
+  {"endpoint": "trip_list", "label": "Trips" },
   {"endpoint": "conference_list", "label": "Conference" },
   {"endpoint": "travel_list", "label": "Travel" },
   {"endpoint": "accommodation_list", "label": "Accommodation" },