move suspended problems list to advanced settings

Moved the list of suspended problems to advanced settings (ie
policy.json) instead of defining them in the <randomize> tag within the
course xml. This avoids potentially destroying student state and makes
it easier for course admins to add/remove suspended problems. Course
admins can now define *all* suspended problems in one place simply by
listing them in the 'Advanced Settings' page of the studio web
interface.