Warn when the in-memory filesystem is getting full

When the internal memory is full, downloads fail without warning and extractions also fail.

GNOME has a notification mechanism for when internal hard disks get full. See https://search.disconnect.me/image?l=aHR0cHM6Ly9lbmNyeXB0ZWQtdGJuMC5nc3RhdGljLmNvbS9pbWFnZXM/cT10Ym46QU5kOUdjUjdDMVE4ZkVvMHlyRnhBcGQwWUFscDBFRmhScVc2WXV0bjNNTU81RldWampXRnZuMTBPUQ==.

We should have something similar in Tails for RAM (which is our internal disk somehow). The message should be custom and recommend to:

• Remove files (downloads, etc.) or move them to the persistent volume.
• Close applications
• Restart

Next step: test how Tails behaves in such situations now that we have !1518 (merged). It might be that we're now killing the app that's allocating too much memory via that tmpfs (and notifying the user that we did it).