Remove Fusion and Fusion-based themes; update to Mix and Match 2.x; add AT Commerce

Since adding Adaptivetheme and several subthemes, we've had two base themes. It's time to drop Fusion and its related modules. Mix and Match now has an AT based version, but we haven't produced a Granada version based on AT. Meanwhile, the AT Commerce theme may be a useful compliment to the AT themes we currently feature.

So:

• Remove Fusion, Fusion Accelerator, and Designkit.
• Remove Granada.
• Update Mix and Match to the 2.x branch.
• Add AT Commerce.
• Provide update instructions for site admins who wish to retain their current versions of Mix and Match or Granada.