Allow the administrator to switch to a different authenticator when AD is not the only authenticator installed

[mflorea]

See:

• https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Authentication/#HChoosetheauthenticator
• https://extensions.xwiki.org/xwiki/bin/view/Extension/AuthServiceBackport/

Comment from @tmortagne :

the AD authenticator should be refactored on rely on the new generic wiki level authenticator choice, but that could be a breaking change for those who already have it (you might have to enable superadmin to set the authenticator back to AD after the upgrade if you don't have local a admin user)