Warn when the persistence is getting full

People are using Tails on tiny devices with no much disk space. Filling up the persistence while working can lead to weird behaviour of Icedove for example (crashing, etc.). We should warn people when their persistence is getting full.

How is this handled by GNOME? See https://search.disconnect.me/image?l=aHR0cHM6Ly9lbmNyeXB0ZWQtdGJuMC5nc3RhdGljLmNvbS9pbWFnZXM/cT10Ym46QU5kOUdjUjdDMVE4ZkVvMHlyRnhBcGQwWUFscDBFRmhScVc2WXV0bjNNTU81RldWampXRnZuMTBPUQ==.

How could we do that?

Related issues

Is duplicate of #10064

Original created by @sajolida on 11863 (Redmine)

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