feat: Integrate wagtailcharts

[mgax]

Fixes https://github.com/ietf-tools/www/issues/236

⚠️ Migrations will conflict with https://github.com/ietf-tools/www/pull/403. Whichever gets merged first, the migrations in the other branch should just be deleted and re-created, they are just a side-effect of the Streamfield schema being changed.

This PR integrates the wagtailcharts library with all its chart types: lines, vertical bars, horizontal bars, area, combo (lines + bars + area), pie, doughnut, waterfall, radar, and polar.

Charts include an extra Accessible label field, which is used as an aria-label attribute on the chart, for screen readers.

---

The branch is deployed at https://ws-wagtailcharts.dev.ietf.org/ for testing.

---

Here's a quick demo, with IETF attendance data, reproducing the chart from this article.