Improve ownership test in tails-debugging-info

Currently, we only print a warning if the file that is going to be printed by tails-debugging-info is not owned by the user that was configured in /etc/whisperback/debugging-info.json.

We could improve it by:

Also checking whether the file is a symlink that is not owned by the correct user
Also check all the parent directories

See https://labs.riseup.net/code/issues/8514#note-58 for a proposed patch for the first point.

Additionally, since we ported tails-debugging-info to Python, we always open the file to be printed as root, while before we opened it as the user which is now configured in /etc/whisperback/debugging-info.json. We should fix that.

Related issues

Related to #8514 (closed)

Original created by @segfault on 15793 (Redmine)

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