(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/client/components/DocComm.ts

@@ -5,7 +5,7 @@ import {ActionGroup} from 'app/common/ActionGroup';
 import {ActiveDocAPI, ApplyUAOptions, ApplyUAResult} from 'app/common/ActiveDocAPI';
 import {DocAction, UserAction} from 'app/common/DocActions';
 import {OpenLocalDocResult} from 'app/common/DocListAPI';
-import {DocUsage} from 'app/common/DocUsage';
+import {FilteredDocUsageSummary} from 'app/common/DocUsage';
 import {docUrl} from 'app/common/urlUtils';
 import {Events as BackboneEvents} from 'backbone';
 import {Disposable, Emitter} from 'grainjs';
@@ -18,7 +18,7 @@ export interface DocUserAction extends CommMessage {
   data: {
     docActions: DocAction[];
     actionGroup: ActionGroup;
-    docUsage: DocUsage;
+    docUsage: FilteredDocUsageSummary;
     error?: string;
   };
 }