Add unambiguous version of flatname to customvar_flat

[julianbrost]

The conversion from complex custom variables to customvar_flat.flatname is lossy, i.e. you can't distinguish vars.some_dict["a"]["b"] from vars.some_dict["a.b"]. Icinga DB Web uses the flat table also for display purposes (as it allows easier implementation of restrictions) but currently has more of a workaround to properly display keys containing dots: Icinga/icingadb-web#762

This could be made easier by adding a separate column that contains information to unambiguously split the flatname into its components, maybe something like a variant where ., [, and ] can be escaped or maybe some JSON list containing the components ["some_dict", "a.b"] or ["some_list", 0].