Support a fullscreen mode in the interactive-figure viewer

[pkgw]

Interactive figures in the AAS journals are limited to fairly small viewports. For image viewing, it could be very helpful to add a fullscreen mode so that people can really explore what's there.

One thing I do not know about is how this might interact with the fact that our viewer is embedded in an iframe. It seems like the kind of thing that might impose some limitations on what we can do. If some kind of cooperation from the containing webpage is required, we can do our best to at least make it easy to implement.