alerts moved under header, no longer overlaying content.

Relates to a UI problem pointed out in https://github.com/PyCon/2014/issues/72 by dianeclarke.

Before:
![before](https://cloud.githubusercontent.com/assets/470417/2822054/4686505c-cf0e-11e3-9ffe-c42dbcd50825.png)
After:
![after](https://cloud.githubusercontent.com/assets/470417/2822056/4bfe24b0-cf0e-11e3-97a8-e8150f90fb2a.png)

I adjusted the messages to be in their own container as needed-- otherwise it'd have to live in the container div underneath, which could be overwritten by a child template by mistake and hide the alerts.