(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/gen-server/entity/Document.ts

@@ -1,4 +1,5 @@
 import {ApiError} from 'app/common/ApiError';
+import {DocumentUsage} from 'app/common/DocUsage';
 import {Role} from 'app/common/roles';
 import {DocumentOptions, DocumentProperties, documentPropertyKeys, NEW_DOCUMENT_CODE} from "app/common/UserAPI";
 import {nativeValues} from 'app/gen-server/lib/values';
@@ -65,6 +66,9 @@ export class Document extends Resource {
   @OneToMany(_type => Secret, secret => secret.doc)
   public secrets: Secret[];
 
+  @Column({name: 'usage', type: nativeValues.jsonEntityType, nullable: true})
+  public usage: DocumentUsage | null;
+
   public checkProperties(props: any): props is Partial<DocumentProperties> {
     return super.checkProperties(props, documentPropertyKeys);
   }