Next trip and previous trip links on trip pages

Closes: #110

templates/macros.html

@@ -3,6 +3,10 @@
 {% macro display_date(dt) %}{{ dt.strftime("%a %-d %b %Y") }}{% endmacro %}
 {% macro display_date_no_year(dt) %}{{ dt.strftime("%a %-d %b") }}{% endmacro %}
 
+{% macro trip_link(trip) %}
+    <a href="{{ url_for("trip_page", start=trip.start.isoformat()) }}">{{ trip.title }}</a>
+{% endmacro %}
+
 {% macro conference_row(item, badge) %}
   {% set country = get_country(item.country) if item.country else None %}
   <div class="grid-item text-end">{{ item.start.strftime("%a, %d %b %Y") }}</div>