Multi-level Navbar Menu

[JamesDoingStuff]

For #52
Couple of things worth mentioning:

• I added the user-event package to make testing keyboard inputs simpler (fireEvent was too restrictive)
• To get the NavMenuLink to work with focus I had to use ref forwarding, and to support router links without duplicating code, it now uses the NavLink, but this meant adding ref forwarding to NavLink as well, lest the console fill up with React warnings.

[akademy]

This looks good.

Just a couple of extra things:

• You'll have to make it work with react-routes types of links - check out how linkComponent prop is used in NavLink.
• Probably fits better in the "src/components/navigation" folder.

I'll review it formally after you get the tests created.

[VictoriaBeilsten-Edmands]

Looks good and you've handles accessibility well. Just a couple of minor points.

[akademy]

@JamesDoingStuff @VictoriaBeilsten-Edmands Any update on this?

[JamesDoingStuff]

I've just noticed that I accidentally committed a bunch of auto-formatting to the changelog, so I'll get rid of that now, but otherwise I think I've made all the suggested changes - just needs approval

[akademy]

LG