ContentMerger now merges topics!

- ContentMerger now requires a TopicMergeStrategy

- Available strategies include:
  - Throw away new topics
  - Throw away old topics
  - Replace topics based on a given equivalence
KM ingest can just replace old topic refs
KM ingest no longer needs to resolve topics

This was originally needed for a check I added to fix an earlier data mistake.
Getty ingest now uses ContentMerger's topic merging
Things have to give ContentMerger a TopicsMergeStrategy
Merge branch 'content-topics-merger'