Indicate current usage period in usage view, and show text when empty

[jldec]

The screenshot below shows usage after clicking on August to show usage for August.

• the currently selected period (August) should have a grey background
• the main list area should say No billed usage for this period.

[svenefftinge]

I opted for adding information about the displayed range, as we should go away from the fixed calendar month views anyway. FWIW I like the usage view in vercel