Save WWT interactive figure

[jsub1]

Addresses #46 by using the pywwt interactive figure work (WorldWideTelescope/pywwt#215). This PR adds the option to save the current view to a zip file with an HTML page and supporting script and data files.

[astrofrog]

@jsub1 - sorry this never got merged! Can you rebase and resolve the conflicts?

[astrofrog]

@Carifio24 - could you see if this can be rebased/wrapped up?

[pkgw]

Note that the save_as_html_bundle infrastructure is still in pywwt for now, but it's based on the much older JS/HTML implementation from before we got everything tidied up. I think the code should still work, but it might have some issues, and it would definitely be nice to pull the pywwt implementation up to use the Research App framework.

[Carifio24]

I played around with this a bit, and while the basic setup save_as_html_bundle still works, it can't export a view that has any layers because the time_decay layer attribute (added after this functionality was implemented) is currently not JSON serializable.

Adding the serialization support should be straightforward, and then this would work until we get around to modifying it to use the research app.

[Carifio24]

@astrofrog With WorldWideTelescope/pywwt#332 and WorldWideTelescope/pywwt#345 this should be working now (it will work with only 332, but won't properly export point sizes). Maybe we want to add some sort of pywwt-version-based enabling? I don't have the proper permissions to rebase this since it's not my branch.