MDL-42505 - Feedback plugin: Inappropriate use of fieldset tags around form and cancel button

Changes:
* Removed the fieldset HTML tags around the Cancel button at the bottom of the form.
* Removed the fieldset HTML tags around the whole form.
* Added missing fieldset tags around groups of radio buttons and checkboxes.
* Removed unordered list wrapping dropdown lists.
* Removed duplicate label for dropdown lists.

Modified Files:
* mod/feedback/complete.php
* mod/feedback/complete_guest.php
* mod/feedback/item/multichoice/lib.php
* mod/feedback/item/multichoicerated/lib.php

Signed-off-by: Michael Milette <michael.milette@instruxmedia.com>