Feature: Mass assign participants to multiple races

[ancadogaru]

It would be very usefull to have the ability when you select a set of participants to be able to assign them to multple races. This is available only when you update a single participant, but if you have 100+ participants that are running in 2 races it becomes a nightmare to seleact each of them and assigning to both race

[segfaultcoredump]

I'll try and roll this into the next release.