Limit descriptions width to ease mixed LTR/RTL text reading

This will prevent, on large pages, the LTR and RTL text to be
far away, on each side of the page. This could happen on channel
and playlists descriptions, when the page is displayed on a large
screen.

assets/css/default.css

@@ -523,3 +523,7 @@ p,
 	unicode-bidi: plaintext;
 	text-align: start;
 }
+
+#descriptionWrapper {
+       max-width: 600px;
+}