(core) Add API endpoint to get site usage summary

Summary:
The summary includes a count of documents that are approaching
limits, in grace period, or delete-only. The endpoint is only accessible
to site owners, and is currently unused. A follow-up diff will add usage
banners to the site home page, which will use the response from the
endpoint to communicate usage information to owners.

Test Plan: Browser and server tests.

Reviewers: alexmojaki

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D3420

app/server/lib/DocApi.ts

@@ -851,6 +851,9 @@ export class DocWorkerApi {
       return true;
     }
 
+    // If Redis isn't configured, this is as far as we can go with checks.
+    if (!process.env.REDIS_URL) { return false; }
+
     // Note the increased API usage on redis and in our local cache.
     // Update redis in the background so that the rest of the request can continue without waiting for redis.
     const multi = this._docWorkerMap.getRedisClient().multi();