[BUGFIX] Fix error when posting as guest

In addition, fix a few duplications and unnecessary calls in the postAction of DiscussionController
[TASK] Adjust commenting form to report errors in Poster
[TASK] Add LLL for flash message displayed when authorisation is needed
[TASK] Render FlashMessages as simple Bootstrap alert, info class
[TASK] Change flash message alert box class to success for higher visibility
[TASK] Add display of flash messages in Write mode