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/factories/messages.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :message do
+    content { 'Message' }
+    status { 'sent' }
+    message_type { 'incoming' }
+    fb_id { SecureRandom.uuid }
+    account
+    inbox
+    conversation
+    user
+  end
+end