Move contestId out from under Candidate in RawContestConfig

[HEdingfield]

This is confusing for two reasons:

1. The actual contestId that's provided by the user exists under the CvrSource class already.
2. It breaks the model of having the RawContestConfig file be a 1:1 mapping of what the .json config files look like.

This appears to only be used for Dominion tabulation, so it should probably be refactored to DominionCvrReader or maybe ContestConfig.