If there’s one thing American television is good at, it’s knowing when to call it a day. That’s why tv shows never overstay their welcome and are always sure to go out on a high note, never dragging on and on, tarnishing any fond memories we might have of its early glory, just trying to squeeze out every possible penny of an idea that is clearly limping along on thrice-recycled ideas. That’s why NBC knows that now is the time to announce a spin-off show of The Office, starring Dwight Schrute called The Farm. Because the people making The Office obviously just have too much original content and too many fresh ideas to fit them all into one show …