CONTRIB-4778 :: In Conditional branching questionnaire, back navigation deletes previous answers.
CONTRIB-4779 :: Do not prevent from navigating to previous page when required questions on page are left unanswered.

Signed-off-by: Joseph Rézeau <moodle@rezeau.org>
Merge branch 'MOODLE_25_STABLE' of https://github.com/sebts/moodle-mod_questionnaire into SEBTS_25