Add skip_updates option

[assuntaw]

This adds a skip_updates option to wrap any database CRUD changes in that should not be broadcast, for example because they are happening in an environment without a running Redis server.

# app/models/application_record.rb
class ApplicationRecord < ActiveRecord::Base
  include CableReady::Updatable

  self.abstract_class = true
end

# db/seeds.rb

# Skips broadcasts for all models inheriting from ApplicationRecord:
ApplicationRecord.skip_updates do
  User.destroy_all
  User.create!(user_attrs)
end

# Skips broadcasts for User only:
User.skip_updates do
  # will not broadcast
  User.destroy_all
  User.create!(user_attrs)

  # will broadcast
  Post.destroy_all
  Post.create!(post_attrs)
end

Big thanks to @leastbad and @julianrubisch for pointing me in the right direction 🤗 I decided to borrow heavily from ActiveRecord's no_touching. Limiting skip_updates to the current thread (rather than working with a class variable) means that we don't skip broadcasts for other models or, crucially, other clients. I think that this is the safest option, and can't really think of a use case for one client prohibiting broadcasts for all other clients.

[julianrubisch]

LGTM!

[leastbad]

I apologize for the delay in getting this approved. I am very impressed with the simplicity of this solution. I would like to understand how it works a bit better so that I'll be able to support and potentially extend it in the future. However, this review is not intended as a blocker and I would be comfortable merging it right away if @julianrubisch is comfortable with everything. My understanding can come as a step 2 in this case. 😸