Properly handle backslashes in example queries

So far, a `\\` in an example query turned into a `\` when selecting the
query. This broke several of the example queries. It is now fixed.

backend/templates/partials/head.html

@@ -64,7 +64,7 @@
   var MIXED_MODE_TIMEOUT = {{ backend.mixedModeTimeout }};
   var examples = [];
   {% for example in examples %}
-  examples.push(`{{ example.query|safe }}`);
+  examples.push(`{{ example.query|addslashes|safe }}`);
   {% endfor %}
   var MAP_VIEW_BASE_URL = "{{ backend.mapViewBaseURL }}";
   </script>