[video recordings] Fixup: generate an external URL

Relates-to commit c83526c.

templates/emails/video-recording-published.txt

@@ -4,7 +4,7 @@ Hi {{ proposal.user.name }},
 
 The video recording for your EMF {{ proposal.human_type }} '{{ proposal.title }}' has been edited and published!
 
-The video and direct links to it can be found at: {{ url_for('schedule.item', year=event_year, proposal_id=proposal.id, slug=proposal.slug) }}
+The video and direct links to it can be found at: {{ url_for('schedule.item', year=event_year, proposal_id=proposal.id, slug=proposal.slug, _external=True) }}
 
 Thank you again for participating in EMF {{ event_year }}!
 {% endblock %}