Replace PHE references with NHS Digital

With the move to NHS Digital, references to PHE need to be replaced across all the files in the project (regexs show case insensitivity):

[ ] Replace /raw.githubusercontent.com/PublicHealthEngland/i with raw.githubusercontent.com/NHSDigital
[ ] Replace /github.com/publichealthengland/i with github.com/NHSDigital
[ ] Replace /\bPHE\b/ with NHSD
[ ] Replace Public Health England with NHS Digital
[ ] Replace PublicHealthEngland with NHSDigital
[ ] Check LICENSE.txt copyright date range is up to date

NHSDigital / ndr_dev_support