Automate the weekly-planning script

The weekly-planning script[1] is currently executed by hand. It should be updated and executed by a cron (or something) the same way the weekly report is managed.

The script needs to be updated a bit for this I believe.

• [1] https://forge.softwareheritage.org/source/snippets/browse/master/swh-team/weekly_planning.sh

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Migrated from T3877 (view on Phabricator)