optimize css loading and split from js dist

__tests__/integration/mirador/embedding.html

__tests__/integration/mirador/index.html

__tests__/integration/mirador/plugins/state.html

__tests__/integration/mirador/window_actions.test.js

__tests__/integration/mirador/window_sidebar.test.js

package.json

src/components/ManifestListItem.js

src/components/WorkspaceMosaic.js

src/styles/index.scss

webpack.config.js