Revamp navbar markup template

[dillix]

The navbar has been entirely rewritten in flexbox with improved support for alignment, responsiveness, and customization.

• Responsive navbar behaviors are now applied to the .navbar class via the required .navbar-expand-{breakpoint} where you choose where to collapse the navbar. Previously this was a Less variable modification and required recompiling.
• .navbar-default is now .navbar-light, though .navbar-dark remains the same. One of these is required on each navbar. However, these classes no longer set background-colors; instead they essentially only affect color.
• Navbars now require a background declaration of some kind. Choose from our background utilities (.bg-*) or set your own with the light/inverse classes above for mad customization.
• Given flexbox styles, navbars can now use flexbox utilities for easy alignment options.
  .navbar-toggle is now .navbar-toggler and has different styles and inner markup (no more three s).
• Dropped the .navbar-form class entirely. It’s no longer necessary; instead, just use .form-inline and apply margin utilities as necessary.
• Navbars no longer include margin-bottom or border-radius by default. Use utilities as necessary.
• All examples featuring navbars have been updated to include new markup.