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