fix incorrect role labels for migrated issues and comments (#22914)

Fix #22797.

## Reason
If a comment was migrated from other platforms, this comment may have an
original author and its poster is always not the original author. When
the `roleDescriptor` func get the poster's role descriptor for a
comment, it does not check if the comment has an original author. So the
migrated comments' original authors might be marked as incorrect roles.

---------

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/issues/comment.go

@@ -1244,3 +1244,8 @@ func FixCommentTypeLabelWithOutsideLabels(ctx context.Context) (int64, error) {
 
 	return res.RowsAffected()
 }
+
+// HasOriginalAuthor returns if a comment was migrated and has an original author.
+func (c *Comment) HasOriginalAuthor() bool {
+	return c.OriginalAuthor != "" && c.OriginalAuthorID != 0
+}