chore: upgrade sequelize (#965)

* 0.18.0

* chore: Upgrade sequelize 4 -> 5

* fix: migrations v5 support

* fix: Majority of test failures

* fix: the rest of v5 tests

server/utils/zip.js

@@ -7,7 +7,7 @@ import { Collection, Document } from '../models';
 
 async function addToArchive(zip, documents) {
   for (const doc of documents) {
-    const document = await Document.findById(doc.id);
+    const document = await Document.findByPk(doc.id);
 
     zip.file(`${document.title}.md`, unescape(document.text));