Updated tinymce code block handling to help prevent breaking history states
Updated tinymce code block handling to help prevent breaking history states

Only used an undo transaction on startup and added a small delay
to codeMirror parsing on SetContent's to help avoid
the rendering activities getting caught in undoManager states.
Seemed to improve things a lot in Firefox & chrome on my dev machine.

For #2602

BookStackApp/BookStack,BookStackApp/BookStack,BookStackApp/BookStack