Aesthetic improvements to preview experience. 

* Loading spinner now uses wagtail's standard glyph
* "preview_placeholder" renamed "preview"
* Page being previewed now loads in an iframe inside the preview page, rather than directly in it. This means a nicer transition is possible once loading is complete.

Having triple-read @gasman's explanations of the pitfalls with loading content in new windows and having checked this in ie9-11, ff, safari and chrome, I'm confident it's working.
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