Port Backend to SQLite/Sequelize (removes MongoDB), Support Electron (#14)

* The MongoDB/Mongoose data storage is removed in favor of Sequelize. This abstracts over SQLite or RDBMs like PostgreSQL and MSSQL. The default is SQLite, which significantly simplifies deployments in end-user environments.

* As Spacedeck now has no more mandatory server dependencies, we can wrap it in Electron and ship it as a desktop application.

* Removes docker-compose.yml

* First version of import UI

middlewares/404.js

@@ -1,6 +1,6 @@
 'use strict';
 
-require('../models/schema');
+require('../models/db');
 var config = require('config');
 
 module.exports = (req, res, next) => {
@@ -16,4 +16,4 @@ module.exports = (req, res, next) => {
   } else {
     res.status(404).send("Not Found.");
   }
-}
+}