Add the ability for Rooms to have a URL.

The URL is shown on the schedule and the event/proposal page.
The URL is only when a user is logged in and registered.

app/views/schedules/_event.html.haml

@@ -17,6 +17,9 @@
     %h4
       - if(event.speakers.any?)
         presented by #{event.speaker_names}
+    - if event.room&.url && @conference.user_registered?(current_user)
+      %p
+        = link_to('Join Event', event.room.url, target: '_blank')
     %p
       = markdown(truncate(event.abstract, length: 400))
       = link_to 'more', conference_program_proposal_path(@conference.short_title, event.id) if event.abstract.length > 400