Fix url prefix usage in attestations

Merged Samuel GAIST requested to merge 558_attestations_fix_url_prefix_usage into django3_migration

This merge request fixes the URL generation in the emails sent to warn users about future deletion of their attestations.

It also fixes an execution issue where the command sending the email would also to the cleanup.

Both command are now independent so their actions are clearer. The daily cron tasks to be executed have been updated and now both of them are called in sequence.

Part of #558 (closed)

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