Work on code/documentation issues to improve the translations dashboard

[hturner]

@benubah, who supervised the Google Summer of Code projects producing the translations dashboard has opened several issues on the translations-dashboard repo with improvements that could be made. The dashboard was also reviewed under #50 leading to a further batch of issues for improvement.

This task is to review the open issues and select one or more to work on. There is a range of issues:

• Data analysis with R: e.g. analyze and visualize translations metadata translations-dashboard#11
• Refactoring R code: e.g. move table codes away from index.Rmd translations-dashboard#8
• Dashboard development: e.g. dashboard architecture translations-dashboard#42
• Project documentation/governance: e.g. add detail to README translations-dashboard#40

[hturner]

Another couple of issues to highlight:

• Main dashboard: only update CSV and HTML when translations change translations-dashboard#46
• Weblate data: only request data since CSV files were last updated translations-dashboard#47