Stats: Fix the graph timeframe when navigating to old periods

[youknowriad]

closes #10955

In this PR, I update the graph timeframe when navigating back to old periods. We make sure the selected date is always inside the graph timeframe.

Testing instructions

• Go to `/stats/month (or /days or /weeks) and select a site if prompted.
• Make sure the site select has more than a year (or more than 30 days) worth of stats.
• Click the left arrow next to the main heading on the page until you navigate past the first bar on the left side of the graph.
• The chart timeframe should update correctly. The selected date should still be visible on the chart (orange bar)

[matticbot]

Test live: https://calypso.live/?branch=fix/stats/old-periods

[timmyc]

Very nice - this is a great addition being able to scroll back in time.