Add root-level Site field to GraphQL schema

[williamjallen]

Feature Request

How can we make CDash better?

#1982 introduced a site field to the Build type and a aggregate sites field to the Project type in our GraphQL schema. It would be helpful to also have a root-level sites field and a corresponding site field to allow API consumers to query sites across all projects they can see. The challenge with this is ensuring that consumers can only access sites which have contributed to projects they have access to, while also remaining efficient. Additional database indexes may be necessary to support this.

This work will assist with #1984.