auto-guess user's t-shirt size

That's something that annoyed me since the beginning: you need to enter
your t-shirt size each time you register to a new conference. So add a
request to fetch the last size entered by the user. We'll guess they
didn't took a size since last time, and if they did, they can always
update the value :)
partial revert of 4c629dd80a427b58d5e066857794c82415381ac6