Web-app: deposit moderation view- use origin url for external-id

Today the moderation view (https://archive.softwareheritage.org/admin/deposit/) generates the url of the external-id on the fly by using /

This is a problem at present with deposits from the hal-preprod platform, which is not an urgent/big problem.

On the other hand, when the deposit will be open to new clients, this property will be problematic as is. That's why the url associated to the external-id should be directly taken from the swh-id-context where the origin url is defined.

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Migrated from T1274 (view on Phabricator)