Configurable sidebar width and preventing overlap of fixed headers

[sled]

I added a variable $mtl-layout-default-sidenav-width which allows to configure the sidebar width. However the primary reason is that in case of a fixed header there was an overlap on the right side due to the combination of width: 100%; and a left padding (sidebar).

The specific use case is something like:

= mtl_header class: 'navbar-fixed' do
  %ul.right
    %li
      = mtl_button 'Save', '#'

In this case the button disappears due to an overflow to the right and is not visible

@marcopluess Do you approve the CSS? 😄

[lwe]

👍 looks good for me

[marcopluess]

👍