Improve system admin permissions to dms and gms (#29656)

* revert the api4 portion of 57372bd06b

* test: Add system admin channel access tests

* skip a.HasPermissionToTeam for channels without a team

* fix TestPostGetInfo

server/channels/app/authorization.go

@@ -278,7 +278,7 @@ func (a *App) HasPermissionToChannel(c request.CTX, askingUserId string, channel
 	}
 
 	channel, appErr := a.GetChannel(c, channelID)
-	if appErr == nil {
+	if appErr == nil && channel.TeamId != "" {
 		return a.HasPermissionToTeam(c, askingUserId, channel.TeamId, permission)
 	}