Move darkMode to Redux

[josephmturner]

This will require adding a darkMode reducer, which takes one single action (toggleDarkMode). This action will not yet be called in any components, but it will be called in a settings modal when we add that.
The darkMode boolean (appState.darkMode) should be pulled into any component via mapStateToProps. The list of components which require darkMode for styling can be found with a simple grep.