Make web security config require spring-security-web on classpath

Previously, SpringBootWebSecurityConfiguration required a web
application and @EnableWebSecurity to be on the classpath.
@EnableWebSecurity is in spring-security-config so this could lead
to the configuration being active when spring-security-web isn't
on the classpath but the rest of Spring Security is.

This commit updates SpringBootWebSecurityConfiguration to also make it
conditional on a class in spring-security-web.

Fixes gh-2717

Author: wilkinsona

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/SpringBootWebSecurityConfiguration.java

@@ -87,7 +87,7 @@
  */
 @Configuration
 @EnableConfigurationProperties
-@ConditionalOnClass({ EnableWebSecurity.class })
+@ConditionalOnClass({ EnableWebSecurity.class, AuthenticationEntryPoint.class })
 @ConditionalOnMissingBean(WebSecurityConfiguration.class)
 @ConditionalOnWebApplication
 public class SpringBootWebSecurityConfiguration {