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/conversations.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :conversation do
+    status { 'open' }
+    display_id { SecureRandom.uuid }
+    user_last_seen_at { Time.current }
+    agent_last_seen_at { Time.current }
+    locked { false }
+
+    factory :complete_conversation do
+      after(:build) do |conversation|
+        conversation.account ||= create(:account)
+        conversation.inbox ||= create(
+          :inbox,
+          account: conversation.account,
+          channel: create(:channel_widget, account: conversation.account)
+        )
+        conversation.sender ||= create(:contact, account: conversation.account)
+        conversation.assignee ||= create(:user)
+      end
+    end
+  end
+end