Improve performance

Problem/Motivation

Drupal CMS's performance test is showing that Mercury loads nearly 300 KB of CSS (unclear how much JavaScript) into each page presented to anonymous users. This is true even outside of the site template.

This will likely negatively affect performance, especially on mobile, so I wonder if there are optimizations we can make here. We could also add a performance test to Mercury itself so that we're never unintentionally adding unnecessary page weight.