Subscription for newly created Contributions

[hero101]

Description

Currently, there is a single subscription for newly created Posts. We should unify newly created Contributions under a single subscription, which will give extra information about the Contribution itself and the underlying entity - link, post, whiteboard

As a user I want live feedback when something new is created in the Callouts So that I am up-to-date with the latest developments

Acceptance criteria

• [ ] There is a subscription to a Callout, about newly created Contributions

Additional Context

There is a subscription for newly created Posts, which is returning the Post and some minimal information about the Contribution.