Central storage location for config and user data

I suggest storing all user config and user data in a central location. This facilitates backup and virtual setups (virtual machine, vps, container #1880, ...) which often mount a single folder only. Most packages provide configuration options to change storage location of configuration files and user data. If any package does not provide such an option, symlinks can be used as an alternative.

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Summary from discussion with @qa-nick-guest and #1992

• Store app-specific data (for multi-user aware apps mostly only configurations etc.) under /home/app/<appname> (And create symlinks to the files appropriately where apps would expect the files.)

• Store regular user-data in the users' /home/<user_name> or /home/group/<group_name> directories (#1992). (And create symlinks to the files appropriately where apps would expect the files.)

• Store all the freedombox configuration files under /home/app/freedombox/<absolute-path-in-rootfs> (to make their location obvious). (And create symlinks to the files ln -s ./<absolute-path-in-rootfs> /<absolute-path-in-rootfs> where they are expected, e.g. under /etc.

Consequently, e.g. the plinth.conf would get included on backup media as <mountpoint>/FreedomBox-<hostname>/home/app/freedombox/etc/plinth/plinth.config.