- Move the sidebar-specific styles into `styles/sidebar` and the
  component-specific styles into `styles/sidebar/components`. This mirrors
  the structure of the JS code in `src/`.

- Rename the entry point for the sidebar's style bundle from "app.scss"
  to "sidebar.scss" to make it more obvious what is in the generated
  "sidebar.css" bundle.

This move highlights some inappropriate dependencies between style
modules and an unclearly structured assortment of "stuff" shared by both
annotator and sidebar bundles in shared files in `src/styles`. There is
probably a fair amount of unused code in there as well. Cleaning that
up will happen in separate commits.