preload user and editor on discussion subentries
preload user and editor on discussion subentries

refs VICE-1793
flag=react_discussions_post

Test Plan:
- Run the following query in graphiql

```
query MyQuery {
  legacyNode(_id: "203", type: DiscussionEntry) {
    ... on DiscussionEntry {
      id
      discussionSubentriesConnection {
        nodes {
          message
          author {
            name
          }
          editor {
            name
          }
          quotedEntry {
            previewMessage
            createdAt
            updatedAt
            author {
              name
            }
          }
        }
      }
    }
  }
}
```

- It should batch the user loads into a single sql statement
- specs pass

Change-Id: I30bfd03335255cda88afded7a775d57af8e444c6
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/272060
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
QA-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Product-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms