Consider replacing use of SiteID with simply an Entity index

[mxgrey]

The prevalence of SiteID throughout the site format/editor implementation and its similarity to builtin Bevy entities forces a significant amount of brittle machinery on our implementation.

The main blocker that prevents us from moving away from SiteID is the fact that if we serialize raw entity indexes then those indexes could clash if we load multiple sites into the same application simultaneously.

To move away from SiteID we would need to choose either:

1. Do not support loading multiple sites now or ever
2. Use a multi-world implementation so that each site is loaded as its own world

I find option (1) very unappealing since there could be legitimate reasons for a user to load multiple sites into the same application at the same time. Future features that we haven't thought of yet may benefit from this capability.

Option (2) is more appealing but I'm not clear on its feasibility at the moment. The last I checked, multi-world applications are not well supported yet in Bevy and there is ongoing work in Bevy's roadmap to support them better.