Fixes#35472.
`Comment.LoadAssigneeUserAndTeam` already has a Ghost user fallback
for a deleted assignee user, but the parallel branch for a deleted
assignee team just swallowed the not-found error and left
`AssigneeTeam` as `nil`. This is inconsistent (the reporter's example
shows `assignee` becoming a Ghost user while `assignee_team` becomes
`null`), and it's also a latent nil pointer bug: other code that
assumes `AssigneeTeam` is set once this function returns without
error will panic.
Added `organization.NewGhostTeam()` / `Team.IsGhost()`, mirroring the
existing `user_model.NewGhostUser()` / `User.IsGhost()` pattern, and
used it in the same fallback branch.
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
aba0eb1749