Tom Coleman 6638f5cfb7 Save the preview frame URL as the story changes.
Sometimes when there are problems with HMR, we end up doing a
hard-reload of the preview iframe, whilst leaving the main window.

As the preview iframe didn't change its URL ever, this led to problems
where the old (usually original) story was loaded in such circumstances.

See #614 and #1328
2017-11-22 17:11:12 +11:00
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