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)
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(cherry picked from commit 7fb032c240)
(cherry picked from commit 64438ff837)
(cherry picked from commit 8174592b29)
