Line Chart: Annotations for important dates

[alykat]

This feature would allow someone to flag important dates during the time span covered by the data in a line chart. (Recent example, though in the template version all annotations should be visible on desktop and mobile: https://apps.npr.org/dailygraphics/graphics/pm_nonres_gdp-20181217/#desktop)

This might exist as a separate sheet in the template spreadsheet that's looped over in the code.

This would be optional, toggled via the content spreadsheet. (See how it's done in the state grid map template.. It would be off by default.

This is different from #305, where individual points on a line are annotated.