This replaces the buttons that aren't part of a macro and that we don't
need to write additional styles for with their govuk-frontend equivalent.
There were some links that were styled to look like buttons, so these
have also been replaced with the new govuk-frontend macro.
There was one button on the `choose-account.html` template that was in a
section of code that was never reached - this has been deleted.
There’s a lot of code in service settings which:
- talks to the API directly through the clients
- passes that information through to the Jinja template
By encapsulating this logic in the service model:
- the Jinja template can access the data directly
- the logic can be reused across multiple methods
