TextualBody: only highlight code block if language was specified

The highlight.js autodetection is finicky and often wrong, so disable
highlighting unless the language was explicitly specified, or if the
user has explicitly enabled it in the settings.

Fixes vector-im/riot-web#508.

Signed-off-by: Johannes Löthberg <johannes@kyriasis.com>

src/components/structures/UserSettings.js

@@ -93,6 +93,10 @@ const SETTINGS_LABELS = [
         id: 'disableMarkdown',
         label: 'Disable markdown formatting',
     },
+    {
+        id: 'enableSyntaxHighlightLanguageDetection',
+        label: 'Enable automatic language detection for syntax highlighting',
+    },
 /*
     {
         id: 'useFixedWidthFont',