Fix: extract mentions when publishing cards

This wasn't working when creation and publishing happened on separated instances
of the same record, which is what happens when you edit a draft and then you
publish it

https://app.fizzy.do/5986089/cards/2835/edit

test/models/concerns/mentions_test.rb

@@ -20,6 +20,8 @@ class MentionsTest < ActiveSupport::TestCase
         boards(:writebook).cards.create title: "Cleanup", description: "Did you finish up with the cleanup, @david?"
       end
 
+      card = Card.find(card.id)
+
       assert_difference -> { Mention.count }, +1 do
         card.published!
       end
@@ -33,6 +35,8 @@ class MentionsTest < ActiveSupport::TestCase
         boards(:writebook).cards.create title: "Cleanup", description: "Did you finish up with the cleanup, #{attachment.to_html}?"
       end
 
+      card = Card.find(card.id)
+
       assert_difference -> { Mention.count }, +1 do
         card.published!
       end