Enable the core/navigation block and style

[Gentyspun]

Ref: #316

Re-enables the core/navigation block, adds a block style for a grey background and styles the block to match the old custom Menu block.

Flickity functionality for the front end is going to be handled at a later date in a separate ticket

Steps to test:

1. Edit a post
2. Open the block inserter and search for navigation (you may need to disable the branding plugin if it doesn't show)
3. Insert the block, and populate, it should match the styles of the old custom Menu block
4. Check and change the block style to grey background
5. Save and view on the front end

Considerations:

• RTL: Needs to be tested for RTL users to ensure the block displays correctly

Example:
http://bigbite.im/i/cnKyZT

[jaymcp]

I'm not sure why, but the frontend renders a different menu to what I expect. I set it to output a sample menu, and the "toxic twitter" menu was instead output during preview. Any ideas?

stale

[Gentyspun]

I'm not sure why, but the frontend renders a different menu to what I expect. I set it to output a sample menu, and the "toxic twitter" menu was instead output during preview. Any ideas?

Hmmm, very strange. I tried to replicate and I see the correct menu

[jaymcp]

I'm not sure why, but the frontend renders a different menu to what I expect. I set it to output a sample menu, and the "toxic twitter" menu was instead output during preview. Any ideas?

Hmmm, very strange. I tried to replicate and I see the correct menu

I wonder if it's because the converted menus are for some reason drafts, barring the "toxic twitter" one? When I select a classic menu, it converts okay, but it's not published. Since one can't view drafts on the frontend under normal circumstances, perhaps it's just pulling in the only published one:

stale