* Extract thread list into its own component for better encapsulation
   and easier testing

 * Rename `annotation-card` to `thread-list__card` and move it to
   the component styling file for `thread-list`.

   Unfortunately a couple of visual effects still require it
   to be referenced in annotation.scss

 * Remove ng-show hack in thread list

   Remove the "ng-show" attribute which was added as a hack for reasons
   which are no longer applicable. See
   https://github.com/hypothesis/h/issues/2642#issuecomment-150629305
   for original context.

 * Remove unused js-hover class and the code that supports it

   It turns out that this class is no longer referenced in code or
   applicable styling.