Chore: Convert Message Sender to polymorphic (#740)

Fixes #680

app/views/api/v1/accounts/conversations/messages/create.json.jbuilder

@@ -7,4 +7,5 @@ json.content_type @message.content_type
 json.content_attributes @message.content_attributes
 json.created_at @message.created_at.to_i
 json.private @message.private
+json.sender @message.sender.push_event_data
 json.attachments @message.attachments.map(&:push_event_data) if @message.attachments.present?

app/views/api/v1/accounts/conversations/messages/index.json.jbuilder

@@ -17,6 +17,6 @@ json.payload do
     json.private message.private
     json.source_id message.source_id
     json.attachments message.attachments.map(&:push_event_data) if message.attachments.present?
-    json.sender message.user.push_event_data if message.user
+    json.sender message.sender.push_event_data if message.sender
   end
 end

app/views/api/v1/widget/messages/create.json.jbuilder

@@ -7,4 +7,4 @@ json.created_at @message.created_at.to_i
 json.private @message.private
 json.source_id @message.source_id
 json.attachments @message.attachments.map(&:push_event_data) if @message.attachments.present?
-json.sender @message.user.push_event_data if @message.user
+json.sender @message.sender.push_event_data if @message.sender

app/views/api/v1/widget/messages/index.json.jbuilder

@@ -7,5 +7,5 @@ json.array! @messages do |message|
   json.created_at message.created_at.to_i
   json.conversation_id message.conversation.display_id
   json.attachments message.attachments.map(&:push_event_data) if message.attachments.present?
-  json.sender message.user.push_event_data if message.user
+  json.sender message.sender.push_event_data if message.sender
 end