Update menu html structure so it's one single hierarchical list

[colinbm]

⚠️ Don't forget to update the gem version in the CHANGELOG before merging! When you're ready to release bump version file and generate a tag. ⚠️

What

Updates the navigation to use a single top level ul and nested subnav.

Why

This is a follow up to #237 with a better solution.

The previous markup was like this:

<ul>
  <li>
    <a href="">Top level link</a>
  </li>
</ul>

<ul>
  <li>
    <a href="">Top level link</a>
    <ul>
      <li>indented nav</li>
    </ul>
  </li>
</ul>

This separates each top level nav item into a separate list, where the nav should be a single component.

New markup is:

<ul>
  <li>
    <a href="">Top level link</a>
  </li>
  <li>
    <a href="">Top level link</a>
    <ul>
      <li>indented nav</li>
    </ul>
  </li>
</ul>

[36degrees]

I don't think b35334d belongs in this PR, but otherwise this seems to make sense to me.

[colinbm]

@36degrees It's more adjacent... would you prefer it be in a separate PR? Or I can edit the title to include it explicitly?

[36degrees]

I think it's more or less the same commit as f46aee7 in #238? At the minute it'll conflict with that PR because it's changing the same lines of code but in slightly different ways.

[colinbm]

Ah so it is - thanks for clarifying - will remove it here.

[36degrees]

Thanks @colinbm 👍🏻