Preferences for passage editor helpers

* Added Twine preference checkboxes for the create passage helper and
the import external images helper.
* Fixed bug where the create passage helper would consider external URLs
as broken links.
* Changed "View Last Build" into "Rebuild and View", which rebuilds the
story before showing the build.
* Added "Paste Passage Here" to the Story Map's right-click menu. Use it
to paste a copied passage to a particular location.