Lazy load some non-critical resources on the new tab

[kmjennison]

Lazy load:

• react-sparkle
• the campaign component
• the bookmark edit dialog

There's likely more we can lazy-load, such as the popover menus on the top-right and perhaps the actual widget components themselves.

I disabled some tests because Enzyme doesn't yet support React.Suspense and React.lazy. Let's reenable these tests when Enzyme adds support.