Link custom action to github webhook actions

[ulferts]

Aims for a simple link between actions in GitHub webhook payloads and custom actions defined in OpenProject.

Acceptance criteria

• [ ] Administration to select a custom action for the currently supported GitHub payload actions
• [ ] Whenever a payload is received that has an action for which a custom action is defined, execute that custom action
  • This has quite some limitations e.g. it is not possible to set a user field like assignee based on the user that triggered the PR action

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https://community.openproject.org/wp/53569