load association for group membership on group type
load association for group membership on group type

This was causing an n+1 query on group memberships because
we were not using association loading.

fixes VICE-1224
flag=none

Test Plan:
- Create two users
- Create a group
- Add only one of the users to the group
- As the user in the group navigate to /graphiql and run the
  following query

```
query MyQuery {
  legacyNode(_id: <group_id>, type: Group) {
    ... on Group {
      id
      name
      member(userId: <user_id>) {
        createdAt
        state
        user {
          name
        }
      }
      membersConnection {
        nodes {
          user {
            name
          }
        }
      }
    }
  }
}
```

- The query should return all memberships in the group
  as well as the specific membership associated with the
  user id passed into the query
- Use the user id of the user not in the group in the query
- The result should be null for the specific member but the
  membersConnection should still return all users in the
  group

Change-Id: I684e74c6fc0fc6a443497fe1b680ea10440e11d7
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/261028
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
QA-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Product-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>

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