Support multiple correct responses in repertoires

[jven]

Feedback from a user:

"I noticed the problems that come with building a repertoire that has multiple responses from yourself, where you basically choose 1 random 'correct' response while studying. I think it would be very powerful to support multiple correct answers per response so that a ton of different variations can be practiced without having to build and switch repertoires. Loving the tool as is though!"

This is a feature that ChessTempo supports in their opening trainer. In study mode, if the user is prompted with a position that has two correct answers, they should be able to play either of them. Then the line would continue from whichever move the user chose. If the user does not choose any correct move, we could hint all of the correct options.