chore: Refactor backlinks and revisions (#1611)

* Update backlinks service to not rely on revisions

* fix: Add missing index for finding backlinks

* Debounce revision creation (#1616)

* refactor debounce logic to service

* Debounce slack notification

* Revisions created by service

* fix: Revision sidebar latest

* test: Add tests for notifications

server/services/revisions.js

@@ -0,0 +1,32 @@
+// @flow
+import type { DocumentEvent, RevisionEvent } from "../events";
+import { Revision, Document } from "../models";
+
+export default class Revisions {
+  async on(event: DocumentEvent | RevisionEvent) {
+    switch (event.name) {
+      case "documents.publish":
+      case "documents.update.debounced": {
+        const document = await Document.findByPk(event.documentId);
+        if (!document) return;
+
+        const previous = await Revision.findLatest(document.id);
+
+        // we don't create revisions if identical to previous revision, this can
+        // happen if a manual revision was created from another service or user.
+        if (
+          previous &&
+          document.text === previous.text &&
+          document.title === previous.title
+        ) {
+          return;
+        }
+
+        await Revision.createFromDocument(document);
+
+        break;
+      }
+      default:
+    }
+  }
+}