Alias back_turned -> from_behind

BUR #40602 has been approved by @nonamethanks.

create alias back_turned -> from_behind

Same thing.

Yeah, this is a textbook example of two tags meaning the exact same thing. There is literally no point in having back turned when we've had from behind serving as the same thing for years with zero difficulty.

The bulk update request #40602 (forum #349804) has been approved by @nonamethanks.