add ::authServer:: header/footer token

[matthewandrews]

We have support for several tokens in header/footer templates e.g. ::loginUrl::, ::headerFooterServer:: etc. However we now need to show links to registration, user account, etc., and at the moment this means that we have different URLs hardcoded in templates in develop and master branches of commonui repos (auth-test vs auth prod), and the branches can't be merged.

Perhaps the simplest way to approach this (to avoid having to add extra config to every app) is to add support for something like an ::authServer:: token, returning the value of the existing security.cas.casServerName config property.