Add UI toggle for dark mode

[zackkrida]

This issue is part of the Dark Mode project: #3592. Please see the implementation plan for additional detail and context.

Important

This issue is blocked by #4306. This issue is also blocked from merge by #4308, which will add the setColorMode method (example name) called by this UI component to actually store the chosen value. This PR can be started prior to that one and/or nearly completed, but drafted, until the method exists.

Behind the feature flag added in #4306, add the new user interface element which toggles dark mode, designed in #4155. Supports choosing between “dark”, “light”, and “system” modes, with “system” as the default.

Also add a TOGGLE_COLOR_SCHEME analytics event, with a playload including the color mode preference chosen by the user. We will use this to assess usage of the toggle and dark, mode, generally.

The actual functionality to store and use this value to change the color scheme is added in #4308. This issue purely adds the presentational component to our user interface and connects it to the method for setting the color scheme.