Truncate ranked ballots at sentinel candidates

[peterrrock2]

Summary

Add cleaning functions that truncate a ranked ballot when one of a specified set of candidates or CVR markers is encountered.

Common examples include "overvote", "undervote", and jurisdiction-specific end-of-ballot markers.

Expected behavior

Given:

RankBallot(ranking=["A", "overvote", "B", "C"])

truncating at "overvote" should produce:

RankBallot(ranking=["A"])

The marker and every ranking position below it are removed.

Part of #337.