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 97667e06b3)
(cherry picked from commit 8ef73a09c9)
(cherry picked from commit fa8a00d264)
(cherry picked from commit 2ada2074b5)
(cherry picked from commit f9a59b940a)
(cherry picked from commit cd82834043)
(cherry picked from commit 7fb032c240)
(cherry picked from commit 64438ff837)
