Database config page for when config is missing.

Fixes #244.

![image](https://cloud.githubusercontent.com/assets/1657047/3034525/97b069ae-e075-11e3-9448-c8752359808e.png)
![image](https://cloud.githubusercontent.com/assets/1657047/3034557/35032c8c-e076-11e3-8d46-90f5b27086c3.png)

I added it to make it clear that we are talking about php variables.