Style tweaks to the app

This makes a few changes to the presentation of the app:

- The header now includes the title of the app, the current environment, and a link to sign out.
- Alert and success messages now display in Bootstrap alert boxes.
- The list of topics are now presented in a table, rather than a list.

![screenshot of the changes](https://cloud.githubusercontent.com/assets/71922/4460910/2126e7a0-48b6-11e4-9302-2abc3f375a06.png)