Mark the DOM as readonly to prevent keyboard popup

You do not want half of your phone screen covered with a keyboard for
readonly files.

Author: Kevinjil

webapp/public/js/domjudge.js

@@ -1477,6 +1477,7 @@ function initDiffEditorTab(editorId, diffId, submissionId, models) {
         scrollBeyondLastLine: false,
         automaticLayout: true,
         readOnly: true,
+        domReadOnly: true,
         theme: getCurrentEditorTheme(),
         renderSideBySideInlineBreakpoint: breakpoint,
         useInlineViewWhenSpaceIsLimited: true,