Fix entity script destruction

[ada-tv]

• Unload scripts that are auto-refreshing before restarting them. (Fixes Entity scripts being automatically reloaded don't call this.unload #1697)
• Refactors entityScriptChanging to account for old and new URLs, was previously always setting them both to "" and failing. (Fixes Replacing client entity script in the Create UI does not "unload" the script, and may not stop it. #2187)
• Cleaned up EntityTreeRenderer::checkAndCallPreload to be less confusing lol

[zedwick]

Things mostly work correctly, but there are a few outstanding edge cases which need some work:

• Replacing the main client entity script url with the same as on the child script entity calls unload on the client entity script but not the script entity script, but does remove both scripts to replace them with one fresh client entity script.
• Deleting the parent entity when it has both a client entity set and a child script entity will only call unload on the script entity's script.

And another minor issue I just noticed, somewhat unrelated:

• Hitting reload after entering a client entity script url but before it has loaded the script will cause it to load the script just to unload it before loading it again.

Everything else seems to work as expected, thanks!