Register an AuthenticationManager in security autoconfig

This is quite a big step, but I think it helps a lot. Since Spring
Boot always creates an AuthenticationManager if it doesn't find one
already registered, it makes sense to also make it into a @Bean.
Spring Security does not register its AuthenticationManager by
default though, so we have to do that for it if the user has created
one with an @Autowired AuthenticationManagerBuilder, but not registered
it as a @Bean.

Having the @Bean (marked @Primary to prevent issues with @Autowired)
makes it easier to reason about what Spring Boot has done for you, and
easier to default in simple use cases to the boot-created
AuthenticationManager. For example, if I want an OAuth2 Authorization
Server with password grant, it makes total sense for the
AuthenticationManager for users to be the same as the @Primary one.
Now it is easy to set that up (just @Autowire it).