make the number of days to look back configurable

[neon-ninja]

This PR lets you adjust how many days are considered when calculating the y axis data. With this you can easily see rougher or smoother lines.

Working demo: https://raw.githack.com/UoA-eResearch/covidtrends/configurable_slopeDays/

[aatishb]

Interesting. A few folks have written in requesting this, and it's definitely interesting to play with the slider. Pro: more customizability for power users. Con: Potentially confuses the graph as people can download variants with different y axes. I need to think a bit more on whether we want to incorporate this. If we do, perhaps it could be in some kind of 'advanced' section so it isn't visible at root level.

[rpkoller]

At first I was reluctant and thought it would be reasonable to limit the lower limit of the days looking back to 4 or 5 days and setting the value up to 14 you would get a smoothened curve. But on a second thought especially the days to look back set to 1 or 2 is a good way to associate spikes with events in real life. So over all a pretty good idea and feature but definitely necessary tp create some sort of "advanced" section. Regular users might get puzzled and lead on the wrong track with it.