rename admin to advanced settings & add help text

closes #163

app/views/servers/_settings_header.html.haml

@@ -7,6 +7,6 @@
     - if Postal.ip_pools?
       %li.navBar__item= link_to "IP Rules", [organization, @server, :ip_pool_rules], :class => ['navBar__link', active_nav == :ip_pool_rules ? 'is-active' : '']
     - if current_user.admin?
-      %li.navBar__item= link_to "Admin", [:admin, organization, @server], :class => ['navBar__link', active_nav == :admin ? 'is-active' : '']
+      %li.navBar__item= link_to "Advanced Settings", [:advanced, organization, @server], :class => ['navBar__link', active_nav == :admin ? 'is-active' : '']
     - if organization.admin?(current_user)
       %li.navBar__item= link_to "Delete", [:delete, organization, @server], :class => ['navBar__link', active_nav == :delete ? 'is-active' : '']