Allow MongoDB credentials to be set

This commit together with openstad/openstad-frontend#354 allows MongoDB to have authentication.

This commit introduces 3 new environment variables:
- MONGO_DB_USER
- MONGO_DB_PASSWORD
- MONGO_DB_AUTHSOURCE

If both `MONGO_DB_USER` and `MONGO_DB_PASSWORD` are set, they are added to the MongoDB connection string.
The `MONGO_DB_AUTHSOURCE` allows us to specify which database we are authenticating against. (see https://www.mongodb.com/docs/manual/reference/connection-string/#std-label-connections-connection-options)

app-init.js

@@ -39,12 +39,19 @@ const sessionStore = new MySQLStore({
 */
 
 
-const mongoCredentials = {
-  host: process.env.MONGO_DB_HOST || 'localhost',
-  port: process.env.MONGO_DB_PORT || 27017,
+function getMongoDbConnectionString () {
+  const host = process.env.MONGO_DB_HOST || 'localhost';
+  const port = process.env.MONGO_DB_PORT || 27017;
+  const user = process.env.MONGO_DB_USER || '';
+  const password = process.env.MONGO_DB_PASSWORD || '';
+  const authSource = process.env.MONGO_DB_AUTHSOURCE || '';
+
+  const useAuth = user && password;
+
+  return `mongodb://${useAuth ? `${user}:${password}@` : ''}${host}:${port}/sessions${authSource ? `?authSource=${authSource}` : ''}`;
 }
 
-const url = 'mongodb://'+ mongoCredentials.host +':'+mongoCredentials.port+'/sessions';
+const url = getMongoDbConnectionString();
 
 const sessionStore =  new MongoStore({
     url: url,