Use DOMContentLoaded instead of load event for CodeMirror initialization. Closes #1894 (#1898)

datasette/templates/_codemirror_foot.html

@@ -5,7 +5,7 @@
   const schema = {};
   {% endif %}
 
-  window.onload = () => {
+  window.addEventListener("DOMContentLoaded", () => {
     const sqlFormat = document.querySelector("button#sql-format");
     const readOnly = document.querySelector("pre#sql-query");
     const sqlInput = document.querySelector("textarea#sql-editor");
@@ -38,5 +38,5 @@
         });
       }
     }
-  };
+  });
 </script>