use partial for referenced <link>s

layouts/partials/util/get_licence_link.html

@@ -0,0 +1,46 @@
+{{- /*
+  Returns the URL for a given licence, either from the site data or from a defined URL.
+
+  @params licences_data Site data of licences, indexed by SPDX IDs.
+  @params licence   Licence identifier.
+  @params licence_link  Manually-defined licence URL. Optional.
+  */
+-}}
+
+<!-- Validation -->
+
+{{- if ( not ( isset . "licences_data" ) ) -}}
+    {{- errorf "No licence data passed to partial" -}}
+{{- end -}}
+
+{{- if ( not ( isset . "licence" ) ) -}}
+    {{- errorf "No licence requested in partial" -}}
+{{- end -}}
+
+<!-- Variable assignment -->
+
+{{- $licences_data := .licences_data -}}
+{{- $licence := .licence -}}
+{{- $licence_link := .licence_link -}}
+
+{{- $final_licence_link := "" -}}
+
+<!-- Rendering -->
+
+{{- if ( index $licences_data $licence ) -}}
+    {{- if eq ( index $licences_data $licence ) $licence_link -}}
+        {{- warnf "Redundant license link '%s' defined, it already exists in the site data" ( index $licences_data $licence ) -}}
+        {{- $final_licence_link = ( index $licences_data $licence ) -}}
+    {{- else -}}
+        {{- if ( and ( $licence_link ) ( not ( eq ( index $licences_data $licence ) $licence_link ) ) ) -}}
+            {{- warnf "Licence '%s' has a different link ('%s') defined in the site data, using '%s' instead" $licence ( index $licences_data $licence ) $licence_link -}}
+        {{- end -}}
+        {{- $final_licence_link = $licence_link -}}
+    {{- end -}}
+   
+{{- else if $licence_link -}}
+    {{- warnidf "licence-link-not-in-site-data" "License link '%s' for licence '%s' should be in the site data, unless it's rarely used" $licence_link $licence -}}
+    {{- $final_licence_link = $licence_link -}}
+{{- end -}}
+
+{{- return $final_licence_link -}}