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_routes.js

@@ -170,7 +170,6 @@ var SpacedeckRoutes = {
               location.href = "/";
             } else {
               this.active_view = "account";
-              this.load_subscription();
             }
           }.bind(this)
         }