(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/migration/1651469582887-DocumentUsage.ts

@@ -0,0 +1,18 @@
+import {nativeValues} from "app/gen-server/lib/values";
+import {MigrationInterface, QueryRunner, TableColumn} from "typeorm";
+
+export class DocumentUsage1651469582887 implements MigrationInterface {
+
+  public async up(queryRunner: QueryRunner): Promise<any> {
+    await queryRunner.addColumn("docs", new TableColumn({
+      name: "usage",
+      type: nativeValues.jsonType,
+      isNullable: true
+    }));
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<any> {
+    await queryRunner.dropColumn("docs", "usage");
+  }
+
+}