LP#1831786: add demo of cross-tab communications

LP#1831786: add demo of cross-tab communications

This adds to the Angular sandbox page a demo of implementing
cross-tab communications using BroadcastChannel. To quote the
instructions added by this patch:

"To test, open this sandbox in a second browser tab.
Enter something in the input box below, then switch to the other tab
and click anywhere on the page. You should see the message that you
sent to the other browser tab."

Sponsored-by: MassLNC
Sponsored-by: Georgia Public Library Service
Sponsored-by: Indiana State Library
Sponsored-by: CW MARS
Sponsored-by: King County Library System
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>