* add subspaces to be listed with edit_hash/spaceAuth authorization
* remove dead code from api_helpers.js
* add edit_hash authorization for requested space thumbnails
* handle /s/:hash links in frontend router
* set space_auth via a function, allow passing it to load_space
* rename variable in /s/:hash router in backend
* hide search, profile, breadcrumb in folders if not logged in, construct links to subspaces differently for anonymous editors
* 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
