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/admin/rooms/_form.html.haml

@@ -11,5 +11,8 @@
     = semantic_form_for(@room, url: (@room.new_record? ? admin_conference_venue_rooms_path : admin_conference_venue_room_path(@conference.short_title, @room))) do |f|
       = f.input :name, input_html: { autofocus: true }
       = f.input :size, label: 'Capacity', input_html: {size: 5}
+      = f.input :url, label: 'URL'
+      %p.small
+        The URL is only visible to registered attendees.
       %p.text-right
         = f.action :submit, as: :button, button_html: { class: 'btn btn-primary' }