[misc] mongodb: use the new db connector by default

mongojs was enabling it by default as well.

services/spelling/app/js/mongodb.js

@@ -1,7 +1,10 @@
 const Settings = require('settings-sharelatex')
 const { MongoClient, ObjectId } = require('mongodb')
 
-const clientPromise = MongoClient.connect(Settings.mongo.url)
+const clientPromise = MongoClient.connect(
+  Settings.mongo.url,
+  Settings.mongo.options
+)
 
 let setupDbPromise
 async function waitForDb() {

services/spelling/config/settings.defaults.js

@@ -9,6 +9,10 @@ module.exports = {
   },
 
   mongo: {
+    options: {
+      useUnifiedTopology:
+        (process.env.MONGO_USE_UNIFIED_TOPOLOGY || 'true') === 'true'
+    },
     url:
       process.env.MONGO_CONNECTION_STRING ||
       `mongodb://${process.env.MONGO_HOST || 'localhost'}/sharelatex`