Introduce accessibility in more depth in ITP

We've noticed in the Piscine that trainees generally aren't familiar with accessibility in dynamic websites.

We should probably introduce some prep material and a workshop on this after they've learnt about the DOM.

This would probably look like an accessibility refresher workshop in Data Flows Sprint 1 where we take their To Do List app from Data Groups Sprint 3 and do some accessibility testing for the stateful app after adding/completing/... some to dos.