ClinGen historical versions

ClinGenAllele responses change over time, but we have built things assuming they are static. We should think about how we handle changes, or whether we want to store historical versions to be able to fully debug how things worked in the past now the data has changed

Split out of SACGF/variantgrid_shariant#199 - which lists some examples of HGVS changing over time (gene symbol appearing)

As we have to make an API call to ClinGen to use it for HGVS resolution, we store the API response updating the ClinGen allele record.

We display created/modified on the Allele liftover page - so you can see when it was created and last retrieved. However I'm thinking maybe we should either:

* Store historical versions
* Add another field - "checked" which is when we did the last API call to ClinGen. We can then only store the api response if it changed, so modified would be last change we retrieved. This could at least tell us when it became different