Modified extension scriptloaders to tolerate firefox active content blocking under https

firefox has introduced "mixed active content blocking" which means
that an exhibit web page loaded with https will refuse to load http: script
links.  At the top level this can be fixed by a user choosing suitable
relative urls for exhibit scripts
(//api.simile-widgets.org/exhibit/...)  .  However, the map and time
extensions load other scripts whose urls were hard-coded to http.
Fixed by setting these urls to https if the document url is https.