Werk #559: WATO snapshots: improved validation of (uploaded) snapshots

Some new validation features have been added

• Only tar and tar.gz files can be uploaded
• It is checked if the file size if reasonable
• It is checked if the content of the tarfile 'looks' OK (real check is done later)

Upon restoring the snapshot, an additional integrity check is done for each of the subtar files. If this check fails, e.g. some tar.gz is corrupted, the snapshot won't get applied and an error message is raised.

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