feat: Document subscriptions (#3834)

Co-authored-by: Tom Moor <tom.moor@gmail.com>

app/models/Subscription.ts

@@ -0,0 +1,29 @@
+import { observable } from "mobx";
+import BaseModel from "./BaseModel";
+import Field from "./decorators/Field";
+
+/**
+ * A subscription represents a request for a user to receive notifications for
+ * a document.
+ */
+class Subscription extends BaseModel {
+  @Field
+  @observable
+  id: string;
+
+  /** The user subscribing */
+  userId: string;
+
+  /** The document being subscribed to */
+  documentId: string;
+
+  /** The event being subscribed to */
+  @Field
+  @observable
+  event: string;
+
+  createdAt: string;
+  updatedAt: string;
+}
+
+export default Subscription;