Core/Commands: Prevent creating accounts containing '@' character with .account create command

sql/updates/world/2016_03_28_00_world.sql

@@ -0,0 +1,3 @@
+DELETE FROM `trinity_string` WHERE `entry`=1030;
+INSERT INTO `trinity_string` (`entry`,`content_default`) VALUES
+(1030,'You are attempting to create a battle.net account with wrong command, use .bnetaccount create');