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).