In Less, the size_class XXS is defined and used in media queries.
This rev. adds it in JS for sake of consistency.
Code in addons comparing the size_class needed to be adapted due
to the new XXS size_class.
Moreover, we now use the helper 'config.device.isMobile' everywhere
we should, instead of manually comparing the size_class.
Finally, we changed the way the config.device has to be specified
in the test environment. From now on, only the size_class should be
set, and the isMobile flag is computed automatically.
Aaron Bohy
cf178a2f06