Created by: paulRbr
When customising your own admin dashboards, it is quite expected to use consistent graphical elements (such as buttons) in custom pages.
Exposing a button--alt CSS class is helpful when adding some custom
buttons, however the existing CSS class includes some spacing
properties which is specific to the navigation bar (because this class
is only used there in the Administrate engine).
This PR moves the spacing property (margin) into a dedicated
.button--nav class in order to give the .button--alt class a
reusable role.