Move to using shinyGovstyle for the external_link() function

[cjrace]

We've added external_link() into shinyGovstyle, so now it's time for dfeshiny to remove the code we have and instead just provide a wrapper around the code that's there, meaning we'll only need to update in one place moving forwards.

Will be a nice first example of something we've developed here and then has grown into a function that's used more widely than DfE so we've been able to pass on to shinyGovstyle instead while still protecting backwards compatibility.