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

app/models/subscription.rb

@@ -36,6 +36,6 @@ class Subscription < ApplicationRecord
   end
 
   def notify_creation
-    $dispatcher.dispatch(SUBSCRIPTION_CREATED, Time.zone.now, subscription: self)
+    Rails.configuration.dispatcher.dispatch(SUBSCRIPTION_CREATED, Time.zone.now, subscription: self)
   end
 end