display account.last_seen_on for admin and moderators

A column has been added with last_seen_on on admin's page "latest
users".

On each user profile, administrators and moderators can view this new
piece of information. User can view it also for itself (that's less
interesting for it, because it should be always "today").

Author: Trim

app/views/admin/accounts/index.html.haml

@@ -16,6 +16,7 @@
     %th Adresse IP courante / précédente
     %th Commentaires / contenus / étiquettes
     %th Date de création
+    %th Dernière visite
     %th Actions
     %th Changer de rôle
   - @accounts.each do |account|
@@ -69,6 +70,7 @@
             #{ image_tag "/images/icones/denied.png", alt: "masqués", title: "masqués", width: "16px" }
           %br
       %td= account.created_at.to_s(:posted)
+      %td= account.last_seen_on ? account.last_seen_on.to_s(:date) : "-"
       %td
         - if account.inactive? && account.user_id != 1
           = button_to "Activer",   [:admin, account], method: :put, class: "ok_button"

app/views/users/_recent.html.haml

@@ -10,6 +10,7 @@
       %li Rôle : #{a.display_role(@user.nodes.where(public: true).count>0)}
       %li Dernière connexion : #{a.current_sign_in_at ? l(@user.account.current_sign_in_at) : "-"}
       %li Dernière action : #{a.updated_at ? l(@user.account.updated_at) : "-"}
+      %li Dernière visite : #{a.last_seen_on ? l(@user.account.last_seen_on, format: :long) : "-"}
       %li Karma : #{a.karma} (minimum : #{a.min_karma}, maximum : #{a.max_karma})
       - if @user.homesite.present?
         %li Site perso : #{@user.homesite}