Revert `update_columns` -> `update!` when rebaking/post-processing post.

`update!` goes through validation which means old posts that doesn't
adhere to the existing validations will raise an error.

app/jobs/regular/process_post.rb

@@ -32,7 +32,7 @@ module Jobs
           # TODO suicide if needed, let's gather a few here first
           Rails.logger.warn("Cooked post processor in FATAL state, bypassing. You need to urgently restart sidekiq\norig: #{orig_cooked}\nrecooked: #{recooked}\ncooked: #{cooked}\npost id: #{post.id}")
         else
-          post.update!(cooked: cp.html)
+          post.update_column(:cooked, cp.html)
           extract_links(post)
           post.publish_change_to_clients! :revised
         end

app/models/post.rb

@@ -603,7 +603,7 @@ class Post < ActiveRecord::Base
     new_cooked = cook(raw, topic_id: topic_id, invalidate_oneboxes: invalidate_oneboxes)
     old_cooked = cooked
 
-    self.update!(
+    update_columns(
       cooked: new_cooked,
       baked_at: Time.zone.now,
       baked_version: BAKED_VERSION