Move new discussion composer to a modal (fixes #44)

I've likely done something atrocious with the CSS. Feel free to yell at me about anything that should be different! The tabs in the dark themes are maybe a little wonky, but I'll leave it to the theme authors to tweak those :)

![screenshot 2014-03-24 02 10 57](https://f.cloud.github.com/assets/376107/2497292/73ee88d4-b334-11e3-90e4-1e4dba329c7e.png)
![screenshot 2014-03-24 02 11 12](https://f.cloud.github.com/assets/376107/2497293/73ef4d82-b334-11e3-8216-4e78722099a5.png)
![screenshot 2014-03-24 02 11 23](https://f.cloud.github.com/assets/376107/2497294/73f22e4e-b334-11e3-8ddc-f8d8562f7e88.png)