consent: New option: showNoConsentAboutService

Since the "about service"-link takes the user away from the IdP, the
user will still have an session on the IdP. This option allows us to
disable that link.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3158 44740490-163a-0410-bde0-09ae8108e29a