* add a validation to prevent an inconsistent combination of values and tests for this validation
* show related method in diff
* remove comment
* extract reusable logic from SalesLogVariables to independent module
* update lettings log tests around derivations related to renewal
* refactor derivation logic to share functionality with sales logs where possible
* remove some tests which are now duplicated above, refactor tests using Jack's wonderful :change suggestion, add in tests about logic deriving vacdays that was not previously covered
* minor changes after tech review
Arthur Campbell