Allow for creation of environmental variables for self-hosting/and instance on a different domain/google cloud project

[samschooler]

Currently the allowedOrigins are hard-coded. This PR adds the ability to customize this and a few other options via env vars.

This was prompted because the re-write removed the ability for a self hosted instance of excalidraw to export links.

Fix in previous version here: excalidraw/excalidraw#600
Relevant code in new version:

excalidraw-store/index.ts

Lines 28 to 52 in f9270d9

	let allowOrigins = [
	"excalidraw.vercel.app",
	"https://dai-shi.github.io",
	"https://excalidraw.com",
	"https://www.excalidraw.com",
	"https://math.preview.excalidraw.com",
	];
	if (!PROD) {
	allowOrigins.push("http://localhost:");
	}
	const corsGet = cors();
	const corsPost = cors((req, callback) => {
	const origin = req.headers.origin;
	let isGood = false;
	if (origin) {
	for (const allowOrigin of allowOrigins) {
	if (origin.indexOf(allowOrigin) >= 0) {
	isGood = true;
	break;
	}
	}
	}
	callback(null, { origin: isGood });
	});

My ideal would be to re-open cors so self-hosted versions can export using the excalidraw.com services.

My second best solution is this PR, coupled with a default removal of the hosted service env vars to avoid confusion.