Ensure paragraphs in trail-text inherit correct trail-text size.

Straight forward refactor. The danger moment in terms of regression is removing the 16px rule from `p`.

It's a positive change though, because `p` isn't a 'thing' we want to have an explicit font size. It should either inherit from its context (eg, trail-text, article-body) or from the `body` element.

@mattandrews @phamann feedback please.