Support chained user providers

Support for chained providers is needed for the remember-me feature to work. 

Since an OpenID provider has to be the first one in the list of providers and it doesn't support any user classes, the remember-me handler just removes the remember-me cookies.

This PR changes the test that is done to determine if the user provider implements the `UserManagerInterface`. Since `ChainUserProvider` is not going to implement that interface but one of the providers in a chain could, the check was too restrictive.