Overhaul the downloads page

This reworks the download page in order to:
* Clarify the difference between stable and playtest builds, and make the playtests a bit more attractive.
* Advertise our FreeBSD and Lutris packages.
* Redirect Fedora / openSUSE / Arch downloads to the better-quality downstream sources.
* Add more details to the platform panels.

![screen shot 2014-11-15 at 16 12 12](https://cloud.githubusercontent.com/assets/167819/5056333/f934ca7c-6ce2-11e4-9309-713cb3db5033.png)