Deprecate Chartit in favor of API and JS charts

[vpetersson]

We used chartit as a quick-win in the early version of Sonar. As it has now been deprecated, it would be desirable to replace it.

Here's where my head is at:

• We expose the metrics over an API (using DRF).
• We then replace the Chartit graphs with something like Chart.js, which will provide us with a lot more flexibility with regards to rendering.

[vpetersson]

Perhaps using Pandas and export them to web is a easier path after all.

[vpetersson]

Alternative tool: https://plotly.com/dash/