Continue adding Nightwatch tests

[openjck]

Some tests that we might want to add:

Interactivity

• The region selectors are present when they should be, and include the correct options
• Metric data changes when the region selector is used
• A date selector appears for the Top Add-ons metric
• The data in the Top Add-ons chart changes when the date selector is used
• A date selector appears for any summary charts
• The data in summary charts change when the date selector is used
• When the region selector is used, the scroll position doesn't change (Fix #156: Don't scroll upon region change #165)
• Various region persistence behavior (Persist Region between page loads #121)

Content

• Metric descriptions are correct
• Metric markers (e.g., "FF57 released") are correct
• Metric legends are correct
• The columns of the Top Add-ons table are correct
• The Rank in the Top Add-ons table goes from 1..10
• All links in the footer work
• The correct annotations appear for each chart
• The correct units are used for each chart
• Verify that the <meta> tags, including the <meta property="og:*" content="..." /> tags, are correct
• Verify that the <noscript> message is correct when JavaScript is disabled
• Verify that the page title is correct in index.html before Helmet modifies it
  • We could try disabling JavaScript in a test before checking the title

Behavior

• The dashboard header (including the title, description, and region selector) stays stuck to the top of the page while the user scrolls