Important dates: room allocation by August 15

I think we should mention this in important dates, as we need to share the confirmed rooms list by August 15 to the hotel.

Only rooms confirmed/booked by this date (either self paid or if we need more rooms than we blocked) will be eligible for the discounted rates.

Just in case we are not able use 90% of the blocked rooms, we might want to release those for extra bursaries instead of paying for unused rooms in advance.

cc @nattie