Get rid of build assets from source control

[Krisell]

Our hosting provider doesn't allow us to run node.js (or any other custom binary) so we currently can't build the frontend on the server and instead include the build assets in source control. Let's find another solution for this, so that source control can be kept clean. Since we do have FTP/SSH access, we could for instance do the build in a GitHub action and send the files directly to the server.