Replace PHE references with NHS Digital

[timgentry]

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