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).

stubs/app/server/lib/globalConfig.ts

@@ -0,0 +1,19 @@
+import path from "path";
+import { getInstanceRoot } from "app/server/lib/places";
+import { IGristCoreConfig, loadGristCoreConfigFile } from "app/server/lib/configCore";
+import log from "app/server/lib/log";
+
+const globalConfigPath: string = path.join(getInstanceRoot(), 'config.json');
+let cachedGlobalConfig: IGristCoreConfig | undefined = undefined;
+
+/**
+ * Retrieves the cached grist config, or loads it from the default global path.
+ */
+export async function getGlobalConfig(): Promise<IGristCoreConfig> {
+  if (!cachedGlobalConfig) {
+    log.info(`Loading config file from ${globalConfigPath}`);
+    cachedGlobalConfig = await loadGristCoreConfigFile(globalConfigPath);
+  }
+
+  return cachedGlobalConfig;
+}