Use a heading and a button for the menu toggles

The <nav> elements are no longer “Untitled nav” in the document outline.
Add Genericons
Add screen-reader-text class to the search form submit instead of using display: none
Don't add a menu item separator in the secondary nav
Add screen-reader-text class to the menu titles
Add a Genericon icon mixin
Decrease the menu toggle font-size
Add hamburger icon to the menu toggles
Improvements to the menu toggle mixin

No longer uses the .menu-toggle class inside the mixin.
Update menu toggle script. The nav is now the grandparent, not parent
Add dropdown toggle script

Child menu item can now be toggled on touch devices!

Styling in a later commit.
Add dropdown toggle LESS variables
Major improvements to the navigation CSS

- Instead of desktop-first styles, the desktop styles and mobile styles
are now separated by their own media queries.
- Added styling for dropdown toggled
- Moved search form styling out of this file (will be added in the next
commit)

This might break some child themes but it’s worth it. I cried each time
I looked at the code before.
Add back the search form styles
Add compiled CSS after navigation changes