Add spring.thymeleaf.check-template property

This commit adds a new `spring.thymeleaf.check-template` property which
is only used for Thymeleaf 3.0+.

Since thymeleaf/thymeleaf#419, the Thymeleaf template resolver
implementations can implement the `setCheckExistence` method - this
enables the template existence verification at **resolution** time,
which means the resolver can return null as a `TemplateResolution`
and let other template resolvers in the chain try.

This new property is set to `true` by default and can be disabled if the
application only has a single resolver and the template existence check
is considered as a performance penalty with the configured resolver.

Fixes gh-6500

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -394,6 +394,7 @@ content into your application; rather pick only the properties that you need.
 
 	# THYMELEAF ({sc-spring-boot-autoconfigure}/thymeleaf/ThymeleafAutoConfiguration.{sc-ext}[ThymeleafAutoConfiguration])
 	spring.thymeleaf.cache=true # Enable template caching.
+	spring.thymeleaf.check-template=true # Check that the template exists before rendering it.
 	spring.thymeleaf.check-template-location=true # Check that the templates location exists.
 	spring.thymeleaf.content-type=text/html # Content-Type value.
 	spring.thymeleaf.enabled=true # Enable MVC Thymeleaf view resolution.