enable system users for comment.LoadPoster (#28014) (#28032)

enable system users for comment.LoadPoster (#28014) (#28032)

Backport #28014 by @earl-warren

System users (Ghost, ActionsUser, etc) have a negative id and may be the
author of a comment, either because it was created by a now deleted user
or via an action using a transient token.

The GetPossibleUserByID function has special cases related to system
users and will not fail if given a negative id.

Refs: https://codeberg.org/forgejo/forgejo/issues/1425

(cherry picked from commit 6a2d2fa24390116d31ae2507c0a93d423f690b7b)

Co-authored-by: Earl Warren <109468362+earl-warren@users.noreply.github.com>