Extracts config.json into its own module (#1061)

This adds a config file that's loaded very early on during startup. 

It enables us to save/load settings from within Grist's admin panel, that affect the startup of the FlexServer.

The config file loading:
- Is type-safe, 
- Validates the config file on startup
- Provides a path to upgrade to future versions.

It should be extensible from other versions of Grist (such as desktop), by overriding `getGlobalConfig` in stubs.

----

Some minor refactors needed to occur to make this possible. This includes:
- Extracting config loading into its own module (out of FlexServer).
- Cleaning up the `loadConfig` function in FlexServer into `loadLoginSystem` (which is what its main purpose was before).

test/gen-server/seed.ts

@@ -605,7 +605,7 @@ export async function createServer(port: number, initDb = createInitialDb): Prom
   await flexServer.start();
   await flexServer.initHomeDBManager();
   flexServer.addDocWorkerMap();
-  await flexServer.loadConfig();
+  await flexServer.addLoginMiddleware();
   flexServer.addHosts();
   flexServer.addAccessMiddleware();
   flexServer.addApiMiddleware();