django: plugins: diffclean

Baptiste Beauplatrequested to merge
lyknode/debexpo:dj/plugins/diffclean into django

Django migration: #47 (closed).

Re-adds the diffclean plugin.

Note: this plugin needs diffstat which is a Depends of lintian. Instead of declaring a new dep for the CI and mentors package, I've cherry-picked the lintian dependency from dj/plugins/lintian.

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