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

public/javascripts/spacedeck_vue.js

@@ -158,7 +158,7 @@ function boot_spacedeck() {
   });
 }
 
-$(document).ready(function(){
+document.addEventListener("DOMContentLoaded",function() {
   window.smoke = smoke;
   window.alert = smoke.alert;