fix(db): fix db auth problem in production (#240)

intends to fix the following error message in production:


```
===
Sun, 27 Oct 2019 11:38:50 GMT, Warning (concise trace)
    at Object.exports.init (/home/adrien/openwhyd/app/models/mongodb.js:163:23)
the options [useUnifiedTopology] is not supported

/home/adrien/openwhyd/node_modules/mongodb/lib/mongo_client.js:462
            throw err
            ^
MongoError: Authentication failed.
    at Function.MongoError.create (/home/adrien/openwhyd/node_modules/mongodb-core/lib/error.js:31:11)
    at /home/adrien/openwhyd/node_modules/mongodb-core/lib/connection/pool.js:497:72
    at authenticateStragglers (/home/adrien/openwhyd/node_modules/mongodb-core/lib/connection/pool.js:443:16)
    at Connection.messageHandler (/home/adrien/openwhyd/node_modules/mongodb-core/lib/connection/pool.js:477:5)
    at Socket.<anonymous> (/home/adrien/openwhyd/node_modules/mongodb-core/lib/connection/connection.js:333:22)
    at Socket.emit (events.js:198:13)
```

app/models/mongodb.js

@@ -144,9 +144,9 @@ exports.init = function(readyCallback) {
   var authStr = '';
   if (authUser && authPassword) authStr = authUser + ':' + authPassword + '@';
 
-  var url = 'mongodb://' + authStr + host + ':' + port; // + "?w=1";
+  var url = 'mongodb://' + authStr + host + ':' + port + '/' + dbName; // + "?w=1";
 
-  console.log('Connecting to ' + url + '/' + dbName + '...');
+  console.log('Connecting to ' + url + '...');
 
   var options = {
     native_parser: true,

scripts/algolia-search-index/mongodb-wrapper.js

@@ -41,9 +41,9 @@ const cacheCollections = function(db, callback) {
 };
 
 const initMongo = (params, callback) => {
-  var url = makeConnUrl(params);
+  var url = makeConnUrl(params) + '/' + dbName;
   var dbName = params.mongoDbDatabase || process.env.MONGODB_DATABASE;
-  console.log('Connecting to ' + url + '/' + dbName + '...');
+  console.log('Connecting to ' + url + '...');
   mongodb.MongoClient.connect(url, MONGO_OPTIONS, (err, client) => {
     if (err) {
       callback(err);