Include the output of mount(8) in WhisperBack bug reports

This would be useful for e.g. #8524 (closed), and more generally for handling every case when it matters whether the user has booted from USB or DVD. It would also provide information about incremental upgrades that were applied, and would help diagnosing persistence issues.

Feature Branch: feature/8719-mount-output-in-bug-reports

Related issues

Blocks #8524 (closed)

Original created by @intrigeri on 8719 (Redmine)

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