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)

While I agree with @elliotcm in the general, it really should be marked up as a list, this is copying prior art so shouldn't stop this pull request from getting merged.

We should talk to @fofr about changing both this and the other examples to list in the future though.