Automation of add forge now workflow

Event	Action
[WebUI] status change to //Accepted//	create a gitlab issue (example: https://gitlab.softwareheritage.org/infra/sysadm-environment/-/issues/4668) - mention the gitlab issue link as a comment of the add forge now request
[sysadmin] scheduling done	change status to //Scheduled// (via web api)
[sysadmin] listing successfully finished	change status to //First listing done// (via web api)
[sysadmin] loading started	change status to //First origin loaded// (via web api)
[sysadmin] Loading successfully finished	send validation email to forge admin (see example below*) using the email feddback tracking system
[sysadmin] validation email sent	close the gitlab issue
Manual ?	update changelog

*Validation email example:

Dear forge administrator,

We are happy to announce to you that your forge has been successfully archived !

The forge repos are now available in the archive : https://archive.softwareheritage.org/browse/search/?q=https%3A%2F%2Fgitlab.univ-lille.fr%2F&with_visit=true&with_content=true

Kind regards, The Software Heritage team

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Migrated from T4678 (view on Phabricator)