feat: Add assignee last seen to conversations (#3069)

- Adds assignee last seen attribute to conversations

db/migrate/20210922082754_add_assignee_last_seen.rb

@@ -0,0 +1,5 @@
+class AddAssigneeLastSeen < ActiveRecord::Migration[6.1]
+  def change
+    add_column :conversations, :assignee_last_seen_at, :datetime
+  end
+end