Change User representation in Webhooks

[jmini]

Fixes #927

The User representation in the Webhooks event is:

{
    "id": 1,
    "name": "Administrator",
    "username": "root",
    "avatar_url": "http://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=40\u0026d=identicon",
    "email": "admin@example.com"
}

Documentation: https://docs.gitlab.com/ee/user/project/integrations/webhook_events.html

This PR introduce a new type EventUser for this simplified representation of User.

Since 6.0.0 is a new major version, I think it is fine to do this breaking change.

All the deserialization tests continue to pass.