Chore: Message to support multiple attachments (#730)

* Changes for the message to have multiple attachments
* changed the message association to attachments from has_one to has_many
* changed all the references of this association in building and fetching to reflect this change

* Added number of attachments validation to the message model

* Modified the backend responses and endpoints to reflect multiple attachment support (#737)

* Changing the frontend components for multiple attachments
* changed the request structure to reflect the multiple attachment structures
* changed the message bubbles to support multiple attachments
* bugfix: agent side attachment was not showing because of a missing await
* broken message was shown because of the store filtering
* Added documentation for ImageMagick

* spec fixes

* refactored code to reflect more apt namings

* Added updated message listener for the dashboard (#727)
* Added the publishing for message updated event
* Implemented the listener for dashboard

Co-authored-by: Pranav Raj Sreepuram <pranavrajs@gmail.com>

app/views/mailers/conversation_reply_mailer/reply_with_summary.html.erb

@@ -9,11 +9,14 @@
         <b><%= message.incoming? ? 'You' : message.user.name %></b>
       </td>
       <td>:
-        <% if message.attachment %>
-          attachment [<a href="<%= message.attachment.file_url %>" _target="blank">click here to view</a>]
-        <% else %>
+        <% if message.content %>
           <%= message.content %>
         <% end %>
+        <% if message.attachments %>
+          <% message.attachments.each do |attachment| %>
+            attachment [<a href="<%= attachment.file_url %>" _target="blank">click here to view</a>]
+          <% end %>
+        <% end %>
       </td>
     </tr>
   <% end %>