Multilevel menus

[akademy]

It should be possible to create multiple menu levels (submenus) in the NavBar and the Footer.

The NavBar could dropdown on hover/select, the footer could display them beneath a heading.

[JamesDoingStuff]

I'd like to have a crack at this - do you have any thoughts on what form the dropdown menu should take? I could do something similar to the 'More' tab on the Diamond website, where the whole bar gets expanded and added to, or I could make it behave more like how a bookmark folder works on the toolbar, with a popup menu?

[akademy]

For the top, NavBar, I was imaging them to work more like your bookmark folder example. I would like them to work focus as well as hover, so that people navigating with a keyboard can still use them. MUI already has some Menu components, not sure whether you can use them here or not.

However, I'm not sold on that, so feel free to experiment if you find something better.

For the footer, I was thinking a flat appearance, no popups, just a lists, grouped together with headings. But that's a lower priority.

[JamesDoingStuff]

I've put what I've done so far on the NavBar menu in #123. I still need to write some tests before it can be reviewed, but feel free to take a look it in the storybook and let me know what you think of it - happy to make changes