Config: Add a configuration page

[wking]

We're storing app state in query parameters, which means we need to preserve those query parameters across links. The form currently only allows users to toggle expanded-ness, which was already available via keybindings. But the new UI is more usable via touch, and it sets the stage for more complicated configuration (e.g. setting GitHub credentials).

The ?back=… handling lets users keep their place as they toggle in and out of the config view. When you leave the config view by toggling the config link, we use the back parameter to send you back to the view you came from. When you leave the config view via another link (e.g. the depviz icon or the jump bar), we need to clear the back parameter. We handle the clearing in changeView, bound to the /-wide onChange. I couldn't find any hooks more tightly bound to the config view, because:

• componentWillUnmount is called before the location changes (so we can't clear back because we might still need it).
• onLeave does not provide access to either the next state or the replace function.

The config form uses controlled components.

The config icon is U+2699 (GEAR).

This commit uses Object.assign frequently for copying objects so we can edit the copy without changing the original. The method is in ECMAScript 2015 (6th Edition, ECMA-262) and is supported on all major browsers except Internet Explorer. If anybody complains about missing support, we can always add the polyfill (assuming react-scripts / babel don't already do that for us).

Fixes #34.

[wking]

Screenshots of the header-bar addition:

And inside the config view:

[wking]

Merged my own PR (over a week without negative reviews).