Add Thymeleaf auto-configuration for WebFlux

Thymeleaf 3.0 implements the Spring 5.0 view infrastructure for WebMVC
and the new WebFlux framework. This commit adds auto-configuration for
the WebFlux support.

In that process, the configuration property for `spring.thymeleaf` has
been changed to add `spring.thymeleaf.servlet` and
`spring.thymeleaf.reactive` for MVC/WebFlux specific properties.

Now that the `spring-boot-starter-thymeleaf` does not only support
Spring MVC, the transitive dependency on `spring-boot-starter-web` is
removed from it.

Fixes gh-8124

spring-boot-samples/spring-boot-sample-web-method-security/pom.xml

@@ -28,6 +28,10 @@
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-security</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-starter-web</artifactId>
+		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-thymeleaf</artifactId>