(core) Broadcast doc usage updates to clients

Summary:
Introduces a new message type, docUsage, that's broadcast to all connected
clients whenever document usage is updated in ActiveDoc.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3451

app/client/components/Comm.ts

@@ -63,6 +63,14 @@ import {Events as BackboneEvents} from 'backbone';
  * @property {Boolean} fromSelf - Flag to indicate whether the action originated from this client.
  */
 
+/**
+ * Event for a change to document usage. Sent to all clients that have this document open.
+ * @event docUsage
+ * @property {Number} docFD - The file descriptor of the open document, specific to each client.
+ * @property {FilteredDocUsageSummary} data.docUsage - Document usage summary.
+ * @property {Product} data.product - Product that was used to compute `data.docUsage`
+ */
+
 /**
  * Event for when a document is forcibly shutdown, and requires the client to re-open it.
  * @event docShutdown
@@ -111,7 +119,7 @@ import {Events as BackboneEvents} from 'backbone';
  */
 
 const ValidEvent = StringUnion('docListAction', 'docUserAction', 'docShutdown', 'docError',
-                               'clientConnect', 'clientLogout',
+                               'docUsage', 'clientConnect', 'clientLogout',
                                'profileFetch', 'userSettings', 'receiveInvites');
 type ValidEvent = typeof ValidEvent.type;