UX: sort admins and moderators by last seen date on /about page

UX: only show active admins and moderators on /about page

app/models/about.rb

@@ -36,11 +36,13 @@ class About
   def moderators
     @moderators ||= User.where(moderator: true, admin: false)
       .human_users
-      .order(:username_lower)
+      .order("last_seen_at DESC")
   end
 
   def admins
-    @admins ||= User.where(admin: true).human_users.order(:username_lower)
+    @admins ||= User.where(admin: true)
+      .human_users
+      .order("last_seen_at DESC")
   end
 
   def stats