When editing a random question, the form will display the tags that the random question
is configured to get questions from even if those tags do not exist anymore.
Editing "random" questions follows a different procedure than adding them.
This commit takes care of editing a "random" question's configuration.
This commit also introduces mod_quiz\form\randomquestion_form, so mod_quiz
can edit random questions without having to use the form that is inside qtype_random.
