Add statistics to visualize and measure the impact of COVID on mapping activities

[piebro]

As mentioned by @matkoniecz at #8 there is a decline/stalling of contributors in the last year(s) (https://piebro.github.io/openstreetmap-statistics/#63f6). This might be related to an increase in the COVID years. It would be interesting to generate plots to investigate the theory. I don't know which kind of data/plots are relevant. Ideas are very welcome.