Incorporate total number of sequences over time

[emmahodcroft]

It would be useful to be able to visualise how (total) sequencing numbers have changed over time in different countries. We already have this data in the perCountryData.json file.

This could be implemented two ways, or a combination of the two:

• A new page ('Sequencing' or similar) that shows graphs of how sequencing has changed over time. Could also be combined with the case counts plot from perCountryDataCaseCounts.json
• Adding an ability to 'overplot' a line showing the total number of sequences to the 'Per Country' and 'Cases' pages. This should be toggleable (somewhere near the top of the page, or floating as you scroll).

PRs welcome!