LP#1797923 Browser client iframe initial loading page

LP#1797923 Browser client iframe initial loading page

Apply a default value to the browser client iframe URL so the iframe
does not attempt to load a copy of the current page.  The default value
points to a simple loading page, displying "Loading..." plus our
standard spinny gif.

Progress gif copied from web/opac/images/ to web/images/ to avoid
dependence on the old opac directory.

Signed-off-by: Bill Erickson <berickxx@gmail.com>
Signed-off-by: Jason Boyer <jboyer@library.in.gov>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>