Add privacy notice links

[cjrace]

Is your feature request related to a problem? Please describe.
We don't have a privacy notice on our apps, and we almost certainly should

Describe the solution you'd like
Would suggest a footer link to the DfE page - https://www.gov.uk/government/organisations/department-for-education/about/personal-information-charter with the text 'Privacy notice'.

Describe alternatives you've considered
Could create custom ones for each app or even for all shiny apps to use but the department is moving towards just centrally managing them so would almost certainly be a waste of time to do anything other than point to the DfE one.

Additional context
Might be worth doing once dfe-analytical-services/shinyGovstyle#77 is resolved and added to shinyGovstyle (I aim to do this semi-soon).