Ensure that mappers have read the instructions before embarking on their first task in a challenge

[mvexel]

This needs discussion

One way to ensure mappers embark on a challenge with confidence is to ensure that they read the instructions for the task.
One way to do this would be to display the instructions once in a modal that the mapper needs to acknowledge before being able to continue. This would happen when the mapper engages with a challenge for the first time. Perhaps when they open the task, perhaps when they are about to open the editor.

One obvious complexity here is dealing with instructions with dynamic content (Mustache tags).