backup: Implement ability to backup abitrary folders

With NAS functionality and sharing, especially using Samba, users can store Desktop/Laptop/Mobile files on their FreedomBox which has a large disk attached. Mobile phone users could dump photos taken using Syncthing too. If FreedomBox takes regular backups of these shared folders, then FreedomBox becomes a backup solution for all the devices on the network.

Add ability to select a folder and take backups of that folder (also regularly #1529 (closed)).