[Enhancement] Fetch previous messages in the conversation (#355)

* Fetch previous messages in the conversation

* Add specs for conversation store

* Fix codeclimate issues

* Exclude specs folder

* Exclude globally

* Fix path in exclude patterns

* Add endPoints spec

* Add snapshots for Spinner

* Add specs for actions

app/javascript/widget/api/endPoints.js

@@ -7,8 +7,9 @@ const sendMessage = content => ({
   },
 });
 
-const getConversation = () => ({
+const getConversation = ({ before }) => ({
   url: `/api/v1/widget/messages${window.location.search}`,
+  params: { before },
 });
 
 export default {