when the relay storage runs full, we add a warning at 80% and another one at 95%, however, there is currently not much the user can do about that ¹, even if they are no longer using multiple devices or exactly know all devices are fine.

first, idea is to avoid that situation as good as possible, eg. delete large files more eagerly on the server, the pre-message will help on that (as eg. text or later thumbnail is still there)

as this is not always possible, however, we want to have an option in the "three dot menu" of connectivity to clear all relays. after internal discussions, it should be a "clear all, everything", so we do not want that per-relay or so. just smth as dc_context_clear_all_relay_storage(context) ^{2 3}