Chore: Convert Message Sender to polymorphic (#740)

Fixes #680

spec/actions/contact_merge_action_spec.rb

@@ -10,7 +10,7 @@ describe ::ContactMergeAction do
   before do
     2.times.each { create(:conversation, contact: base_contact) }
     2.times.each { create(:conversation, contact: mergee_contact) }
-    2.times.each { create(:message, contact: mergee_contact) }
+    2.times.each { create(:message, sender: mergee_contact) }
   end
 
   describe '#perform' do

spec/factories/messages.rb

@@ -9,7 +9,7 @@ FactoryBot.define do
     account { create(:account) }
 
     after(:build) do |message|
-      message.user ||= create(:user, account: message.account)
+      message.sender ||= create(:user, account: message.account)
       message.conversation ||= create(:conversation, account: message.account)
       message.inbox ||= create(:inbox, account: message.account)
     end

spec/lib/integrations/slack/send_on_slack_service_spec.rb

@@ -20,7 +20,7 @@ describe Integrations::Slack::SendOnSlackService do
       expect(slack_client).to receive(:chat_postMessage).with(
         channel: hook.reference_id,
         text: message.content,
-        username: "Contact: #{contact.name}",
+        username: "Agent: #{message.sender.name}",
         thread_ts: conversation.identifier,
         icon_url: anything
       )

spec/models/conversation_spec.rb

@@ -254,7 +254,7 @@ RSpec.describe Conversation, type: :model do
         conversation: conversation,
         account: conversation.account,
         inbox: conversation.inbox,
-        user: conversation.assignee
+        sender: conversation.assignee
       }
     end
     let!(:message) do
@@ -279,7 +279,7 @@ RSpec.describe Conversation, type: :model do
         conversation: conversation,
         account: conversation.account,
         inbox: conversation.inbox,
-        user: conversation.assignee,
+        sender: conversation.assignee,
         created_at: 1.minute.ago
       }
     end