Sharing fixes, Logo fix for add to home screen

A few fixes here:
(1) The show_image function was not returning URLs properly because of the way it tried to access the asset_url function from outside of a view. Moved the function from levels_helper to application_helper to fix that problem (thanks, Laurel!). This means that the images we use for facebook/twitter sharing are now correct (they were failing a falling back to a generic logo image)
(2) The logo image PNG icon used when you add one of our pages to the home screen on iOS contained an alpha channel, which messed up the logo so it looked like one big black rectangle (instead of the expected four keyboard keys are C, O, D, and E). Saved a new _noalpha version of the logo with white in the background and switched to using that in the meta tag.
(3) Changed a string in block and in dashboard to reference the new term "Play Lab" when sharing Play Lab apps.