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

docker-compose.yml

@@ -1,34 +0,0 @@
-version: '2'
-services:
-  sync:
-    image: redis
-  storage:
-    image: minio/minio
-    environment:
-      - MINIO_ACCESS_KEY=AKIAIOSFODNN7EXAMPLE
-      - MINIO_SECRET_KEY=wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY
-    ports:
-      - 9123:9000
-    command: server /export
-  db:
-    image: mongo
-  spacedeck-open:
-    environment:
-      - env=development
-      - MINIO_ACCESS_KEY=AKIAIOSFODNN7EXAMPLE
-      - MINIO_SECRET_KEY=wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY
-    build: .
-    volumes:
-      # - ./:/usr/src/app
-      - /usr/src/app/node_modules
-    command: npm start
-    ports:
-      - 9666:9666
-    depends_on:
-      - db
-      - sync
-      - storage
-    links:
-      - storage
-      - db
-      - sync