Remote configuration fetching.

Because Roblox keeps botching any legible way for me to automatically deduce how to assemble builds of Roblox Studio, I have switched to a remote solution managed from this repository's Config folder. At some point I may even switch to something script-based to avoid hardcoding entirely, but for now this should suffice.

OAuth2Config.json and AppSettings.xml are also now pulled remotely, in case changes are made to either of them. In particular OAuth2Config needs to be a static file because Roblox started distributing branch-specific versions of the file on sitetest1 and sitetest2.