Banner on v2/v3 for upcoming change

[adriancollier]

In order to prewarn users that we are about to make a big change, it makes sense for us to have some sort of banner on the existing v2 code that tells people we're going to be doing something.

The Adobe Froms Central post at the moment is a good example of this: https://www.acrobat.com/en_gb/products/formscentral.html

We should make sure that we have sufficient online resources to point users to from this notification before we make this live.