Review the way logos and text are added to the header

[cjrace]

We had some issues with dashboards that had inserted the DfE logo into the header function, around contrast styles. Not sure if it's an error that was made when creating the dashboard or not, though should review how the function works and if there's anything we can do in this package to help prevent it happening.

A second issue was also identified around a lack of descriptive text for the links out of the header.

I have specific examples I can share on this to have a look at from our accessibility audit.

[mzayeddfe]

fix shinyGovstyle header issues