This recreates the Zooming Image Tool template for the HTML block.  It
does it in such a way that doesn't depend on any external resources:
both the loupe code and sample image are inlined.

Some benefits to this version are:

* We can now maintain the loupe javascript code properly

* Because the javascript is included in the contents of the block
  itself, the course author can customize it as needed

* As opposed to the previous iteration, the magnified image URL is now
  optional: if it's not present, the regular image will be used for
  magnification

* There can now be two or more instances of the tool in the same unit.

This also removes some CSS left over from the previous iteration.