Add can_leave property to Graphql and children.json APIs

What does this MR do and why?

Needed for #556528

We want to add a Leave project quick action to the project list views. This MR adds a canLeave field to the GraphQL and internal REST APIs that are used on the list views.

Changelog: added

References

Screenshots or screen recordings

How to set up and validate locally

GraphQL

1. Go to http://gdk.test:3000/-/graphql-explorer
2. Run the following query

query {
  projects {
    nodes {
      id
      nameWithNamespace
      userPermissions {
        canLeave
      }
    }
  }
}

Internal REST API

1. Go to a group with a project in it
2. In the dev tools network tab inspect the groups/<group path>/-/children.json request

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