New default stylesheet

[iangreenleaf]

We should set a new default stylesheet for new users that feels simple, clean, and modern, and has excellent responsive capabilities. I think Ever So Simple would be a good candidate. We'll need to:

• Bring it into the official Plans styles (and make sure the licensing is in order)
• Update it with any responsive improvements it needs
• Give it a thorough compatibility test for different browsers and devices to make sure it degrades gracefully and all that
• Start setting it to the default sheet for all new users.