This change ensures persistent data is stored in XDG_DATA_HOME
instead of XDG_CACHE_HOME on linux.
It adds the UserData() method on locations.Provider to return a path
suitable for storing persistent data. On linux, this returns
$XDG_DATA_HOME/protonmail (likely ~/.local/share/protonmail), and on
non-linux this returns os.UserConfigDir() because that is assumed
to be a more persistent location than os.UserCacheDir().
locations.Locations has been modified to use this new data directory;
gluon and logs are now stored here.
