Add Backstabbr recorders to board seeding.

[jasonmastbaum]

Make it so it randomizes as it currently does with an option to try to spread the people willing to do Backstabbr recording around.

[UEWBot]

We'd need to add a boolean attribute to TournamentPlayer to say whether the player is willing to sandbox. It could just be used to add bias to the seeder (ideally with a lower weight than players the TD has asked to keep apart). Not sure whether that would work for all use cases (too few sandboxers, just the right number, plenty).

[UEWBot]

Adding the flag and displaying it in the seeded boards would be helpful to TDs, at least