Navbars

Drop support for .navbar-search. We dropped the .navbar-search form layout, so it doesn't make sense to have this anymore. Also, this is just a slightly fancier .navbar-form, which we plan on keeping.
Overhaul styles of default navbar and its sub-components:
Dropdown menu carets (those attached to the actual menu, not the indicators) have been removed so that dropdown menus sit flat against the edge of the navbar.
Navbar vertical dividers have been brought in a smidge, meaning they do not extend the full height of the navbar.
No more box-shadow or gradients on the navbars.
Height of navbar has increased from 44px to 62px for mobile devices, and 50px for desktops.
Removed .navbar-inner and moved relevant styles to .navbar. The inner div is no longer required, greatly simplifying our shit.
Changed .navbar > .nav to .navbar-nav.
Change .btn-navbar to .navbar-toggle.
More consistent naming schema
Redesigned it a bit, lightening the styles and widening it, while also positioning it absolutely to the top right.
Updated navbar brand component:
Changed .brand to .navbar-brand for a more consistent naming schema.
Center aligned .navbar-brand in mobile views, with a max-width: 200px; to limit hit area given a likely navbar toggle will be very near by.
Dropped navbar dividers.
Added support for disabled nav links.
Includes new variables like @navbar-link-color-disabled and @navbar-link-bg-disabled, along with the inverse options.

See https://github.com/twbs/bootstrap/pull/6342