Extend Node to 3 weeks

[adamblanchard]

We have extra time in the course - nice!

Node has the opportunity to extend from two sessions to three. This is good, because it's quite a packed module right now, and it is of course an important one for the backend specialisation.

The exact scope and how we restructure week 3 is up for discussion as part of this ticket. I have two suggestions:

1. We add api docs (specifically swagger). We are requiring it in two projects in the backend specialism, one of those projects will run along side this module. They aren't learning about it officially in our curriculum yet, so this could be the ideal place.
2. Review other existing learning goals and see which we can be better realised by spending more time on them (either going deeper, or just spending more time learning/practicing it), e.g. authentication, security.

This task is done when:

1. New week 3 content (readme, session plan, prep, assignment) is complete
2. Any restructure required for the other weeks is complete