registration: add a field for shoe size

the local team is planning to include havaianas flip flops (which are a very typical brazilian thing) in the swag kit, so we need a shoe size field in the registration form.

https://en.wikipedia.org/wiki/Havaianas

The sizes listed at their website are as follows:

BR      Europe    USA_W     USA_M     UK
33/34   35/36     4.5/5.5   -         2/3
35/36   37/38     6/7       -         3.5/4.5
37/38   39/40     7.5/8.5   6/7       5/6
39/40   41/42     9/10      7.5/8.5   6.5/7.5
41/42   43/44     11/11.5   9.5/10    8.5/9
43/44   45/46     -         11/12     10/11
45/46   47/48     -         12.5/13   11.5/12
47/48   49/50     -         14/14.5   13/13.5