cubicweb #1903304 [bfss] store the path to bfss storage independently of the file path itself [in-progress]

as of now, the file path is absolute, hence non portable across deployments

priorityimportant
typeenhancement
appeared in<not specified>
done in3.20.0
load1.000
load left1.000
closed by<not specified>
patch[storage] store relative paths instead of absolute ones (closes #1903304) [in-progress]