Adjust 'Sign in' button for GitLab's CSRF protection

GitLab introduced CSRF protection for authentication requests in
571ba5a7. The 'Sign in' button has been
adjusted to send a POST request.

app/views/layouts/header/_public.html.haml

@@ -3,7 +3,7 @@
     .header-content
       - unless current_controller?('sessions')
         .pull-right
-          = link_to "Sign in", user_omniauth_authorize_path(:cas3), class: 'btn btn-sign-in btn-success'
+          = link_to "Sign in", user_omniauth_authorize_path(:cas3), method: :post, class: 'btn btn-sign-in btn-success'
 
       %h1.title= title