mark internal users in the admin panel

and don't offer to impersonate them

app/views/admin/users/_head.html.haml

@@ -2,11 +2,13 @@
   = @user.name
   - if @user.blocked?
     %span.cred (Blocked)
+  - if @user.internal?
+    %span.cred (Internal)
   - if @user.admin
     %span.cred (Admin)
 
   .pull-right
-    - unless @user == current_user || @user.blocked?
+    - if @user != current_user && @user.can?(:log_in)
       = link_to 'Impersonate', impersonate_admin_user_path(@user), method: :post, class: "btn btn-nr btn-grouped btn-info"
     = link_to edit_admin_user_path(@user), class: "btn btn-nr btn-grouped" do
       %i.fa.fa-pencil-square-o