Allow Admin change to s_vi Visitor ID Cookie Length

[#1]

Given the stricter rules around Cookies in the EU, it makes sense to make the length of the Persistent Visitor ID cookie easier to change on the Admin side. At the moment, you have to do it with a somewhat clunky change to the s_code as per KB article 1651

This isn't particularly practical for companies with many s_codes, or, you'd think, best practice