Prevent dangling user redirects (#21856) (#21859)

- Backport #21856
- It's possible that the `user_redirect` table contains a user id that
no longer exists.
  - Delete a user redirect upon deleting the user.
- Add a check for these dangling user redirects to check-db-consistency.

models/user.go

@@ -87,6 +87,7 @@ func DeleteUser(ctx context.Context, u *user_model.User) (err error) {
 		&user_model.Setting{UserID: u.ID},
 		&pull_model.AutoMerge{DoerID: u.ID},
 		&pull_model.ReviewState{UserID: u.ID},
+		&user_model.Redirect{RedirectUserID: u.ID},
 	); err != nil {
 		return fmt.Errorf("deleteBeans: %v", err)
 	}