Fix "cancel" on stage edit page should revert all changes

Fixes https://github.com/AUTUpStageTeam/UpStage2013/issues/91 - also
changed the way the preview/unassign media from stage edit page
functions - now media is not unassigned and player access levels are not
changed until stage is saved. Still has some issues: - cancel button
only works once to clear the select boxes, and unsaved changes to select
boxes are not dropped when user navigates away from page.