Cleanup refactor around hash fields in Change/ChangeInput

[shantstepanian]

A few non-ideal things in the current code:

• Change has the original content field; should not be needed. Only there for the acceptable hashes field
• contentHash is a field in Change (meant for persisting to db). We calculate acceptableHashes separately. They happen to be calculated in the same way, but ideally the fields should come from the same place
• rollbackContent should not be used in the calculation classes; only "convertedRollbackContent"