Spacedeck 6.0 CI WIP; more style, UX cleanups; fix account dialog; add more color swatches; new landing page

Releases should be tested and built via CI/CD.

fix session token/cookie handling for arbitrary server IPs; fix realtime update distribution via websockets

* 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