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 97667e06b384d834a04eaa05e8f91563481709b1)
(cherry picked from commit 8ef73a09c9cb08e536aff7df8fc715c8d48c7c4f)
(cherry picked from commit fa8a00d26420ef56497203a5c607b3d336feb813)
(cherry picked from commit 2ada2074b56f3cd981dbacac0a7b1033ff486ffd)
(cherry picked from commit f9a59b940a2ab2c9abac7c346da24b22d15017f1)
(cherry picked from commit cd82834043c14a679cc55eba980c27549b2f87e5)
(cherry picked from commit 7fb032c2400386d6edba769452e1735f32ac3f78)
(cherry picked from commit 64438ff837f411656c0456864bd9be2c47f37311)
(cherry picked from commit 8174592b298eb2f3a34ed67a836739cc9c9e5a42)
(cherry picked from commit 46d36555f9)
00c8b16b8c