Update the header and menu modal with the conditionals for the new header structure

[andersnoren]

This PR builds on the menu location updates @carolinan made in #247, and updates it with the conditionals and fallbacks specified in #246. It also updates the style to match the design.

I couldn't find a way to replicate the wp_nav_menu sub menu button toggles and chevron icons in wp_list_pages without including a custom walker that modifies start_el. If we can find a way to do this without using a custom walker, that would be much, much better.

In addition to the things in that issue, it includes:

• Text labels for the header toggles (fixes Icon-only controls are not accessible #93)
• Close button in the modal menu (fixes A11y: display close button in the menu panel #27)

[pattonwebz]

I am seeing a little strangeness on the admin bar when search/menu appears.

Also a couple clicks on menu ends up putting it into an open but not visible state for some reason.

[andersnoren]

@andersnoren I've updated the modal styles to include a top offset equal to the height of the .admin-bar, but it won't work until the JavaScript is updated with a different scroll lock method. Since the header toggles are always at the top of the screen, we can simply scroll to the top and set the body to overflow: hidden when a modal is shown, at which point the modals will be positioned correctly.

As for the toggles, yes, the JavaScript handling that should definitely be rewritten as well :)

[pattonwebz]

@andersnoren did you mean to mention yourself or were you trying to mention me? lol I am going to look a bit at javascript stuff now so will try see if we can resolve some of that before end of day :)

[andersnoren]

@pattonwebz A bit quick there 😛 Awesome!

[joyously]

I couldn't find a way to replicate the wp_nav_menu sub menu button toggles and chevron icons in wp_list_pages without including a custom walker that modifies start_el.

I've had this problem before and I'm sure a lot of theme authors have this problem. Could we make a patch for core with a filter or something in the right place so we don't all have to write custom walkers?

[pattonwebz]

So I tested the javascript PR at #163 and it does seem to be working. I was unable to properly test merging it with this PR but I think there are some conflicts we need to spend time tomorrow working though so we can get both this and the javascript swap PR into master