Use spl (space left) util instead of trailing space for spacing

Replace trailing space with [Sass util class for spacing](https://github.com/codecombat/codecombat/blob/843ed97643bd1bb78a9482e6e04d0c6125e77370/app/styles/base.sass#L280-L283). 

Related issue [#1303](https://github.com/codecombat/codecombat/pull/1303#issuecomment-48848067).

![screen shot 2014-07-14 at 3 53 08 pm](https://cloud.githubusercontent.com/assets/2450760/3568269/29778b2e-0b2d-11e4-8947-6134a6ed8440.png)