Add iframe schedule widget

[rixx]

Less experienced users can be frustrated with the pretalx schedule widget, as setting its width is not obvious, and frequently the CSS used on their website actively interferes with the widget (e.g. by overriding margins in the schedule boxes, doing odd things to line height, etc).

For isolation, iframes are a cheap and easy option. We would

• add a schedule view that contains just the schedule widget
• include an option in the widget builder explaining the difference between the two ("make sure the schedule appears as expected" vs "I want to mess with the styles and I now what I am doing")
• add width options (for both modes, come to think of it, fixing default scroll behaviour with the embeddable widget)