Improve Sandcastle load time

[pjcozzi]

By using the built Cesium.js for the deployed version.

See #4889 (comment)

[emackey]

In the interest of avoiding thrashing, it's worth noting that we went the other direction (from pre-built to separate requires) back in #2164. We did this so users didn't need to run combine before running Sandcastle. Maybe we could look for the built version for speed, and fall back on the requires for convenience? That seems like a lot of dependency logic to inject just to get a version number and autocomplete working.

[emackey]

+1 for improving load times though!

[pjcozzi]

@emackey I'm suggesting that we basically just have this optimization for the released version. I'm open to other variations, but do not want to require building often.

[emackey]

Ah, release Sandcastle should use a built copy of Cesium, yes.