Add a cron to purge MediaWiki cache

We're starting to use templates and the DynamicPageList on the public wiki. To apply changes to templates or categories, pages needs to be edited or have their cache manually purged (by adding ?action=purge to the URL)

Running this as a cron: https://www.mediawiki.org/wiki/Special:MyLanguage/Manual:purgeList.php would mostly fix this issue.

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Migrated from T2263 (view on Phabricator)