(core) Show usage banners in doc menu of free team sites

Summary:
Also fixes a minor CSS regression in UserManager where the
link to add a team member wasn't shown on a separate row.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3444

stubs/app/client/component/DocUsageBanner.ts

@@ -0,0 +1,10 @@
+import {DocPageModel} from 'app/client/models/DocPageModel';
+import {Disposable} from 'grainjs';
+
+export class DocUsageBanner extends Disposable {
+  constructor(_docPageModel: DocPageModel) { super(); }
+
+  public buildDom() {
+    return null;
+  }
+}