Move MathJax directive to a shared directory

MathJax parsing now on learn pages, and soon to be on the blog, so move
out of the `ide/chat` directive.

services/web/public/coffee/ide/chat/index.coffee

@@ -2,6 +2,6 @@ define [
 	"ide/chat/controllers/ChatButtonController"
 	"ide/chat/controllers/ChatController"
 	"ide/chat/controllers/ChatMessageController"
-	"ide/chat/directives/mathjax"
+	"directives/mathjax"
 	"filters/wrapLongWords"
 ], () ->