check-po-msgstr: Add option to sanitize .po files (#17661)

We want to use the bin/check-po-msgstr script to run checks before building the staging website. To make sure the Ikiwiki build will not break on problematic files, we came up with the solutoin of sanitizing .po files by removing problematic translations from them.

This commit adds the --sanitize option to such script, which replaces problematic translations with empty strings.

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