fix: Temporarily fix text/plain email rendering (#9653)

This is hacky fix for plain text email rendering. The issue happens only
for the text/plain only emails. If there was an HTML component, then the
rendering works fine.

**How was this tested?**

Mac Email client allows you to send text/plain emails. I've sent one to
myself and imported it on Chatwoot. I've also verified that the email
contains only text/plain part.

Sample rendered email below.

<img width="476" alt="Screenshot 2024-06-18 at 8 15 10 PM"
src="https://github.com/chatwoot/chatwoot/assets/2246121/0c3c07f6-c49d-401a-bba5-a79e82b57bd6">

Fixes https://github.com/chatwoot/chatwoot/issues/9649
Fixes https://github.com/chatwoot/chatwoot/issues/9480

app/javascript/dashboard/components/widgets/conversation/Message.vue

@@ -257,7 +257,16 @@ export default {
         html_content: { full: fullHTMLContent } = {},
         text_content: { full: fullTextContent } = {},
       } = this.contentAttributes.email || {};
-      return fullHTMLContent || fullTextContent || '';
+
+      if (fullHTMLContent) {
+        return fullHTMLContent;
+      }
+
+      if (fullTextContent) {
+        return fullTextContent.replace(/\n/g, '<br>');
+      }
+
+      return '';
     },
     displayQuotedButton() {
       if (this.emailMessageContent.includes('<blockquote')) {