do not create duplicate subscriptions

apps/threads.lua

@@ -146,7 +146,12 @@ app:post("thread", "/:slug", function(self)
   end
   
   if self.params.subscribe == "on" then
-    Subscriptions:create({user_id = user.id, thread_id = thread.id, last_seen = os.time()})
+    local subscription = Subscriptions:find({user_id = user.id, thread_id = thread.id})
+    if subscription then
+      subscription:update({last_seen = os.time})
+    else
+      Subscriptions:create({user_id = user.id, thread_id = thread.id, last_seen = os.time()})
+    end
   end
   
   return {redirect_to = self:url_for("thread", {slug = thread.slug}, {page = last_page}) .. "#latest-post"}