Explicitly disable github @mentions

Also remove doubled strikethrough option.

Signed-off-by: Nikola Forró <nforro@redhat.com>

webroot/js/message.js

@@ -23,10 +23,10 @@ class Message {
 			emoji: true, 
 			openLinksInNewWindow: true, 
 			tables: false, 
-			strikethrough: false, 
 			simplifiedAutoLink: false,
 			literalMidWordUnderscores: true,
 			strikethrough: true,
+			ghMentions: false,
 		}).makeHtml(this.body);
 		const linked = autoLink(markdownToHTML, {
 			embed: true,