refactor templates layout

@see https://gohugo.io/templates/new-templatesystem-overview/

layouts/_partials/scripts/show_post_locations_on_map.js

@@ -0,0 +1,46 @@
+{{- /*
+    Renders a post's locations on a map.
+
+    @params pc Page context.
+    @params locations_data Site data of locations.
+    @params locations   Post locations.
+    */
+-}}
+
+<!-- Validation -->
+
+{{- if ( not ( isset . "pc" ) ) -}}
+    {{- errorf "No page context passed" -}}
+{{- end -}}
+
+{{- if ( not ( isset . "locations_data" ) ) -}}
+    {{- errorf "No location site data defined" -}}
+{{- end -}}
+
+{{- if ( not ( isset . "locations" ) ) -}}
+    {{- errorf "No post locations defined" -}}
+{{- end -}}
+
+<!-- Rendering -->
+
+<script>
+    var map = L.map('map').setView([55, -3], 13);
+
+    L.tileLayer('https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
+        maxZoom: 19,
+        attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a>'
+    }).addTo(map);
+
+    var latLons = [];
+    {{ $locations_leaf_nodes := partialCached "util/get_location_leaf_nodes.html" .locations_data -}}
+    {{- range .locations -}}
+        {{- with ( index $locations_leaf_nodes ( replaceRE `, [A-Z]+` "" . ) ) -}}
+            L.marker([{{ .lat }}, {{ .lon }}]).addTo(map);
+            latLons.push([{{ .lat }}, {{ .lon }}]);
+        {{- else -}}
+            {{- erroridf "missing-lat-lon" "Could not find lat-lon for %q" . -}}
+        {{- end -}}
+    {{- end }}
+
+    map.fitBounds(latLons);
+</script>