Stack one single SquashFS diff when upgrading

That is, address a number of UX problems in our upgrade process with an hopefully cheap and temporary solution.

Blueprint: https://tails.boum.org/blueprint/Endless_upgrades/#single-squashfs-diff

Feature Branch: feature/15281-single-squashfs-diff, iuk:feature/15281-single-squashfs-diff, perl5lib:feature/15281-single-squashfs-diff

Subtasks

• #15279
• #15282 (closed)
• #15283
• #15284
• #15285
• #15286
• #15287
• #15288
• #15407 (closed)

Related issues

• Related to #11131 (closed)
• Related to #15290
• Related to #15291 (closed)
• Related to #15277
• Related to #11679 (closed)
• Related to #7499
• Blocks #8415
• Blocks #16209

Original created by @anonym on 15281 (Redmine)