(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/requestUtils.ts

@@ -21,7 +21,7 @@ export const TEST_HTTPS_OFFSET = process.env.GRIST_TEST_HTTPS_OFFSET ?
 const INTERNAL_FIELDS = new Set([
   'apiKey', 'billingAccountId', 'firstLoginAt', 'filteredOut', 'ownerId', 'gracePeriodStart', 'stripeCustomerId',
   'stripeSubscriptionId', 'stripePlanId', 'stripeProductId', 'userId', 'isFirstTimeUser', 'allowGoogleLogin',
-  'authSubject',
+  'authSubject', 'usage'
 ]);
 
 /**