make "require subscription" toggleable in build (#6144)

## Summary

Adds build time feature flags system starting with a flag that indicates
whether subscription is required to use the app. This is only used on
cloud.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6144-make-require-subscription-toggleable-in-build-2916d73d3650813bb140c5e96bcce1ce)
by [Unito](https://www.unito.io)

global.d.ts

@@ -5,6 +5,11 @@ declare const __ALGOLIA_APP_ID__: string
 declare const __ALGOLIA_API_KEY__: string
 declare const __USE_PROD_CONFIG__: boolean
 
+type BuildFeatureFlags = {
+  REQUIRE_SUBSCRIPTION: boolean
+}
+declare const __BUILD_FLAGS__: BuildFeatureFlags
+
 interface Navigator {
   /**
    * Used by the electron API.  This is a WICG non-standard API, but is guaranteed to exist in Electron.