Generate charts for data visualization

[mgxd]

Would be a new endpoint, with basic authentication.

considerations for visualization libraries:

• https://dc-js.github.io/dc.js/
• https://www.chartjs.org/

[mgxd]

Started in #69, but lots more to do:

Visualization

• More charts:
  • Total / version-specific usage per week
  • Error rates / % of error types

Querying

• Create indexes on extensively used columns
• Filter out docker users from unique count (can make the total unreasonably high)