add forge now : change "send message to the forge" template when already sent

after the first message has been sent to the forge admin( with the actual mailto template), the mailto template should propose a different body text, with only the basic mentions, for example :

subject : Software Heritage archival request for XXXXXXX

body : A new message is waiting for your answer : ... please use "reply all"...

Migrated from T4328 (view on Phabricator)

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