Support html and css in Github Gists

[lilleyse]

Fixes #4125

[emackey]

Does this break compatibility with old gists?

[lilleyse]

Oh good point, it does. I'll fix that.

[lilleyse]

Updated

[emackey]

I still think it's unfortunate that Sandcastle treats gists as proprietary data storage, rather than saving a working example to the gist. I think Sandcastle should save to the gist almost exactly what it offers for download, with the exception that (a) the references to Cesium should be via CDN rather than relative links, and (b) the JS file could be broken out to a separate file, as it is on this branch, so long as the HTML file references the JS file. Done correctly, we could easily enable Bl.ocks.org URLs to display working Sandcastle demos, outside of Sandcastle.

That said, if the team wants to merge this as a quick fix for lack of HTML save on gists, I'm not opposed.

[pjcozzi]

I don't know the details here, but if this is too hacky, I would rather wait for a solid implementation since this isn't widely requests (albeit useful).

[lilleyse]

The change here is just mirroring the existing system, I think it's ok to go in now until we restructure things.