Include responsive_tables 2.x in Panopoly profile (but not as a required dependency)

The 2.x branch of the responsive_tables module is a backport of the responsive tables functionality in Drupal 8. Including this would be in line with Panopoly's inclusion of navbar and responsive_bartik.

I'm proposing that we pull it in via the panopoly_theme.make file, and enable it by default in the Panopoly profile, but don't have any of our modules actually depend on it - so, that it's optional and can be disabled. This is important, as it would allow a distro or site to decide to handle responsive tables in the theme or with another module, as there is still no consensus on how to handle them.

All that said, I'm not 100% sure about adding a new module to Panopoly! We could just "officially" recommend it. I'd like to do some more testing with it, especially with how it'd interact with Radix and other Bootstrap-based themes (which provides its own responsive table solution), and think/discuss some more.