Changed t-p-s numeric UID breaks persistence.conf access rights check

Originally created by @intrigeri on #7343 (Redmine)

I installed Tails 1.0, setup persistence, put some files in there. Then upgraded to a build from current experimental, restarted. Then persistence.conf was disabled, as the ACL for that file gives access to the UID the t-p-s user had on Tails/Squeeze.

The fix seems easy: create this user with the expected, fixed UID at ISO build time.

Feature Branch: bugfix/7343-static-uids

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Blocks #7338 (closed)

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