Keep theme in sync with state across the app

[andrewn]

This moves the logic for keeping the theme class on the body in sync with the redux store to a single place, App.jsx. This simplifies how non-IDEView pages can become theme-aware.

I have verified that this pull request:

• has no linting errors (npm run lint)
• is from a uniquely-named feature branch and has been rebased on top of the latest master. (If I was asked to make more changes, I have made sure to rebase onto master then too)
• is descriptively named and links to an issue number, i.e. Fixes #123