Supporting own sync server e.g. rmfakecloud

[Utopiah]

By default the device synchronizes with reMarkable and Google server. This is not always convenient, possible or ethically desired. The project https://github.com/ddvk/rmfakecloud allows to sync to one own server instead. Bringing URLs from /src/remarkable.ts to a configuration file with reMarkable and Google servers as default would allow to conveniently change to any synchronization backend.