Webhook based notifications of long-running user tasks

Add support for webhook based notifications of long-running user tasks:

• deposit
• vault cooking
• save code now
• add forge now
• ...

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Migrated from T3955 (view on Phabricator)

added Roadmap 2022 Web app meta-task priority:Normal labels

changed the description

added priority:High label and removed priority:Normal label

marked this issue as related to swh-webhooks#5 (closed)

marked this issue as related to swh-webhooks#4 (closed)

marked this issue as related to swh-webhooks#3 (closed)

marked this issue as related to swh-webhooks#2 (closed)