Refactor Conversation model (#134)

* Add Conversation factory with dependent factories

* Include FactoryBot methods in rspec config

* Add unit tests for public methods of Conversation model

* Move Current model into a separate file in lib folder

* Disable Metrics/BlockLength rule for db/migrate and spec folders

* Get rid of global $dispatcher variable

* Create Message#unread_since scope

* Refactor callback methods in Conversation model

* Create Conversations::EventDataPresenter

* Add translation keys for activity messages

* Add pry-rails gem

* Refactor Conversation#notify_status_change

* Add mock_redis for test env

spec/rails_helper.rb

@@ -30,6 +30,7 @@ rescue ActiveRecord::PendingMigrationError => e
   exit 1
 end
 RSpec.configure do |config|
+  config.include FactoryBot::Syntax::Methods
   # Remove this line if you're not using ActiveRecord or ActiveRecord fixtures
   config.fixture_path = "#{::Rails.root}/spec/fixtures"