Consider removing top margin from basic typography blocks.

While working on the hmn.md redesign, I'm finding the need to remove top margins from paragraphs within builder blocks. A strategy I've found helpful in order to address these types of issues is to adopt single-direction margin declarations. The idea is best explained in this [CSS Wizardry article](https://csswizardry.com/2012/06/single-direction-margin-declarations/), but here are a few benefits (excerpted from the article).

* Easier to define vertical rhythm in one fell swoop.
* More confidence in (re)moving components if you know their margins all push in the same direction.
* Components and elements don’t have to necessarily live in a certain order if their margins aren’t dependent on adjoining sides.
* Not being concerned with collapsing margins means one less thing to worry about.

@sambulance, @mattheu – what would you think about adopting this for our base typography styles?