Add help text for slug field (bug 1030569)

Left-aligned help text to have it work better with `.boring-text`-style fields:

![screen shot 2014-06-30 at 12 39 54 pm](https://cloud.githubusercontent.com/assets/23885/3433297/00412814-007e-11e4-8984-a9de9eaee4bf.png)
![screen shot 2014-06-30 at 12 38 32 pm](https://cloud.githubusercontent.com/assets/23885/3433298/017fe63e-007e-11e4-9f2c-e9854e701334.png)