Bug 1037868 - Create gaia-sim-picker component

I wasn't happy with having to include all of this twice (the other time in ```apps/sharedtest/test/unit/elements/gaia_sim_picker/gaia_sim_picker_test.js```), but I couldn't think of a way to combine them. Maybe we could move all of the ```require()```s into a file in ```shared/test/unit/```?