Merge branch 'ff-markdown-buttons-fix' into 'master'

Fixed markdown buttons in FF

## What does this MR do?

The markdown buttons weren't working with the `execCommand` function. So it has to fallback to replacing the value in the textarea.

See merge request !5085

app/assets/javascripts/lib/utils/text_utility.js.coffee

@@ -49,8 +49,9 @@
       insertText = "#{startChar}#{tag}#{selected}#{if wrap then tag else ' '}"
 
     if document.queryCommandSupported('insertText')
-      document.execCommand 'insertText', false, insertText
-    else
+      inserted = document.execCommand 'insertText', false, insertText
+
+    unless inserted
       try
         document.execCommand("ms-beginUndoUnit")