Use "header" template part inside of the "header-large-dark" template part

[kjellr]

Fixes #54.

This potential solution is thanks to a conversation I just had with @jasmussen. Rather than create a separate template part for the navigation block, we can actually just the the more generic header.html template part within the header-large-dark.html template.

When folks edit the navigation inside there, it'll update the original header.html template, and keep things in sync.

The only issue I'm seeing is that the Site Title color doesn't appear to inherit the correct white link color (even though it's properly set on the container). I also tried setting it on the template part itself, but that doesn't seem to fix it either. Seems like a Gutenberg bug?

Editor	Front end

To test:

1. Using the site editor, edit the navigation while viewing the front-page template.
2. Save your changes.
3. Ensure that change is synced to the navigation on other templates as well.

[jasmussen]

Nice one. I'll see if I can push a separate fix got the site title color inheritance to the block editor, so it probably shouldn't block this PR.

[jffng]

Ensure that change is synced to the navigation on other templates as well

This aspect of the PR works as advertised. I did run into a visual bug though where the nav links are stacking strangely, but perhaps that's something to address separately:

[kjellr]

That's weird! Are you seeing that without this PR too?

[jffng]

Are you seeing that without this PR too?

Yes, so maybe let's 🚢 and follow up.

[jasmussen]

What a beautiful theme. And by the way, this header template will look so much better once this PR lands:

That said, I can't reproduce the issue. I'm correctly seeing the white color on the frontend and in the editor:

I'm on latest gutenberg trunk. It's possible it's been fixed there inadvertently?