Add an optional URL for each room

This could be used, for instance, to add a link to a map showing the
location of the room.

At present, this will change slots of type "talk" or "tutorial"; if
the first room for the slot has a URL, the room will be presented as a
link to that URL with just the first room's name. If the first room
has no URL, the talk will be presented with a comma-seperated list of
all the rooms used by this slot.

For LCA2018 we have no talks/tutorials with >1 room on the slot, so
this point is moot.