Summary of Changes

This simple PR adds a close button to the JS message system, just like the one that exists for the php message system.

This applies to all js messages (error, warning, success or info messages).

Testing Instructions

1. Use latest staging
2. Go to Menus -> Message -> Add new menu item and Save without any field, you'get a js erro message.
   
3. Apply patch, refresh browser cache and repeat the same process, you'll now get a close button in the message (test it)
   

Test also on frontend js messages (ex: email remainder or other).