Migrate to CSS modules

CSS modules are pretty clearly the future. Migrate from monolithic application.css to CSS modules.

Currently feeling ambivalent about using direct CSS module import (supported by webpack) vs. something like babel-plugin-react-css-modules.

In general this should be relatively straightforward since the BEM blocks defined in application.css roughly correspond to React components. Ultimately we will want to just go one component at a time, migrating all of the rule sets used by classes in that component into the component’s companion CSS module.

We will want to be vigilant for situations in which there is coupling between styles in different components: for example, a situation in which a parent component defines a flex container, but the flex items are inside a child component. Generally we’ll want to refactor so that either the container is moved into the child component, or the flex items are moved down into the parent component; components should be self-contained from a styling standpoint.

Start by reading application.css and mapping its BEM blocks to the corresponding React components. Compare the direct CSS module import approach with babel-plugin-react-css-modules, then identify styling dependencies between parent and child components. Done means the monolithic stylesheet has been migrated component by component without cross-component styling coupling.