[#70102664, #70959884]: mobile sharing page fixes

(1) Remove footer and padding on mobile sharing scenarios for flappy/bounce/studio
(2) Add helper methods to game.rb so we can reduce the number of places we do hardcoded checks against game.app in the rest of the code
(3) Treat studio like flappy & bounce with respect to sharing
(4) add a string and an image needed for social networking sharing of studio

code.org/studio redirect link will be coming soon. It is currently working on staging.