Changed "passage" HTML attribute to "data-passage"

* Renamed the "passage" HTML attribute to "data-passage".
* Removed the "imgpassage" custom HTML attribute. Instead,
"data-passage" serves that role whenever it's used on an <img>,
overriding the linking.
* The Story Map now recognises data-passage attributes, and draws link
arrows in response to them.
* The body text element in the passage editor pane now has initial
focus.
* Fixed an image editor pane crash.