Status Chart: Merged PR line smoothing and bar chart

[StephanTLavavej]

Implement merged PR line smoothing

The comment in calculate_sliding_window() explains the rationale and technique. index.html explains the technique briefly.

Add bars to Monthly Merged PRs

• gather_stats.js
  • Extract write_generated_file().
  • Implement write_monthly_table(). This is very fast (and doesn't need a progress bar) because it iterates through each PR and increments values in a map indexed by year-month.
  • The monthly_table records complete months, so 2019-10 is the first month, and 2020-07 is currently the last month.
  • Because the chart is now a daily-line/monthly-bar combo, the bars need to be given specific dates. I'm positioning them at the 16th of each month (approximately the middle).
  • I'm using merge_bar as the key, to avoid confusion with merged in the daily_table.
• index.html
  • Load monthly_table.js.
  • Rename the line to 'Line: Sliding Window'.
  • Add the dataset for the bars. All we need to do is mark it as type: 'bar'.
  • I'm setting borderWidth: 1 because I think it looks nice.
  • In the x-axis used for all charts, set an explicit max date (to the final value in the daily_table). This is necessary because the bar chart ends earlier - e.g. currently its final date is 2020-07-16. Without an explicit max, hiding the line causes the chart to readjust the x-axis to end when the bars end, at 2020-07-16. This is confusing, and cuts off half of the final bar. With an explicit max, the x-axis remains unchanged when hiding the line.
  • Display the legend for the mergeChart, now that we have 2 datasets.
  • Update and clarify the Explanation.

Other changes

• Handle deleted users by assuming that they were contributors. (We don't have any yet, but vcpkg did.)
• weekly_table.js: 2020-08-07 update.
• Update usernames_contributors.txt.
• Regen daily_table.js, monthly_table.js.

Live preview: stephantlavavej.github.io/STL/