Add CSS and Tabbed Layout to Panel

Since there are two different things being displayed by the devtools, it would make sense to have a tabbed layout in the panel to switch between the Page Context JSON and the Server Error Logs. Having a tabbed layout would be good for future additions to the devtools.

(Another possibility would be to surface the error logs into the console or [panel sidebar](https://developer.chrome.com/extensions/devtools_panels#type-ExtensionSidebarPane). But that depends partially on the outcome of #1 and what format the error logs are surfaced in.)

Along with this issue, some CSS and UI icons should be added. I'm thinking of including the [Blink source](https://github.com/sebmarkbage/blink-devtools) for this, to match the native look of the rest of the Chrome DevTools. The [Facebook React DevTools](https://github.com/facebook/react-devtools) do this, and Paul Irish recommends it, but there isn't much documentation about it.