Show all available variables dynamically (#1315)

* Show all available variables dynamically

* Ran formatter

src/components/GrampsjsFormExternalSearch.js

@@ -386,7 +386,11 @@ class GrampsjsFormExternalSearch extends GrampsjsObjectForm {
           ${this._(
             'Enter a search URL with template variables for person data. Available variables:'
           )}
-          <strong>{{name_given}}, {{name_surname}}, {{place_name}}</strong>
+          <strong
+            >${Object.keys(this.data)
+              .map(key => '{{' + key + '}}')
+              .join(', ')}</strong
+          >
         </div>
         <md-outlined-text-field
           id="custom-name"