feat: Add message content to mention email template (#2838)

app/views/mailers/agent_notifications/conversation_notifications_mailer/conversation_mention.liquid

@@ -1,8 +1,18 @@
-<p>Hi {{user.available_name}}</p>
+<style type="text/css">
+  .quoted-text--content {
+    box-sizing:border-box;
+    padding:8px 16px;
+    margin:0 0 20px;
+    font-size:14px;
+    border-left:5px solid #eeeeee
+  }
+</style>
 
+<p>Hi {{user.available_name}}, </p>
 
-<p>Time to save the world. You have been mentioned in a conversation</p>
+<p>You've been mentioned in a conversation. <b>{{message.sender_display_name}}</b> wrote:</p>
+<blockquote class='quoted-text--content'>
+  {{message.text_content}}
+</blockquote>
 
-<p>
-Click <a href="{{ action_url }}">here</a> to get cracking.
-</p>
+<p><a href="{{ action_url }}">View Message</a></p>