Replace vaccine names with snomed codes

[inglesp]

Vaccine names (Vaccination.VaccinationName) change, which is a pain. We have discussed replacing them with snomed codes in this Slack thread. Anywhere else? Could @HelenCEBM or @LisaHopcroft help?

As an aside, do vaccine IDs (Vaccination.VaccinationName_ID) also change when vaccine names change? I've added a question to Issues to be discussed with TPP.

There is a separate issue for documenting the vaccine names notebook.

[inglesp]

In converting this to an issue, I appear to have taken the credit for @iaindillingham's issue!

[HelenCEBM]

I'm pretty sure we've already been assured that VaccinationName_ID doesn't change and I haven't seen this happen at all. And in fact TPP have suggested that they are more stable than SNOMED codes - switching to SNOMED codes may present new problems (yay!), but these aren't documented afaik