Image preview scrollbars, and other fixes

* Added scrollbars to images that allow you to see the full image, and
added the ability to resize the image preview window.

* Added a new colour for story passages which have no outgoing links -
these are usually "dead ends", or snippets which are <<display>>ed.

* Fixed a bug where zero-length tooltips could appear for empty
passages.

* Jonah and Sugarcane: Made it so that raw HTML <a> links in stories are
styled as if they were proper externalLinks, in case the author
unwittingly used the former in ignorance of the latter (a not uncommon
occurence).