Clear extra fields when changing items through the schedule view

[drnlm]

Should close #300

[drnlm]

This changes create, since that's hit when we drag a talk or page from the side page into the schedule, as those items don't have a data-scheduleitem-id property, which is what's checked for in edit_schedule.js .

update should be hit when dragging items from one position to another in the schedule. That doesn't currently work for we with any browser, but the default update behaviour of copying all the fields is probably correct there.