Team members query GraphQL resolvers

[cbart]

Part of #47043

This pull request adds resolvers for querying team members:

• team/members(first,after,search)
  • totalCount
    • 🚨 countDeeplyNestedTeamMembers is not supported yet.
  • pageInfo
  • nodes
    • 🚨 user querying is not implemented efficiently to keep pull request size low
    • TODO: Follow up with another pull request that fetches users together with team members.
  • 🚨 search parameter is not tested for to keep the pull request size in check (it is already too long).

The implementation leverages ginger-cake pattern based off teams connection resolver.

• There is a pending action item in the ticket to unify compute and applyArgs across connections.
• Perhaps better: leverage graphqlutil.ConnectionResolver?

Test plan

Just one test case with graphQL query for team members' usernames with pagination.

This required however pretty substantial changes in test library:

• Need to now fake database.UserStore with many users, so implemented a fake for that.

• Going down that rabbit hole - needed to apply that to faking admin actor/user (I think it is not too bad).

• Setting up a context for an admin actor looks like this now:

  ctx := userCtx(fakeUsers.newUser(types.User{SiteAdmin: true}))

  I think this reads better than somewhat opaque (see true), and heavily-destructuring:

  ctx, _, _ := fakeUser(t, context.Background(), db, true)

• While setup now takes advantage of global variables (YOLO), to save from superfluous destructuring.

  Before:

  func TestFoo(t *testing.T) {
    // would require another return value now
    db, ts := setupDB()
  }

  After:

  var (
   db *database.MockDB
   fakeTeams *fakeTeamsDb
   fakeUsers *fakeUsersDB
  )
  //...
  func TestFoo(t *testing.T) {
    setupDB()
  }