chore(templates): refactor 403/500 error templates to be more 'serious'

discuss_data/templates/403.html

@@ -5,5 +5,5 @@
 {% block content %}
 <h1>Forbidden (403)</h1>
 
-<p>CSRF verification failed. Request aborted.</p>
+<p>Request aborted.</p>
 {% endblock content %}

discuss_data/templates/500.html

@@ -3,9 +3,7 @@
 {% block title %}Server Error{% endblock %}
 
 {% block content %}
-<h1>Ooops!!! 500</h1>
-
-<h3>Looks like something went wrong!</h3>
+<h1>500 Internal Server Error</h1>
 
 <p>We track these errors automatically, but if the problem persists feel free to contact us. In the meantime, try refreshing.</p>