Customize Thymeleaf default template resolver order

Currently, the default TemplateResolver had no specific order. Thymeleaf
handles that with a "always first" strategy (that can be confusing if
several TemplateResolver have a "null" order.

While it is a fine default (and changing it could lead to weird side
effects), it has to be changed as soon as another TemplateResolver bean
is defined in the project.

The `spring.thymeleaf.template-resolver-order` property has been added to
control  the order of the default TemplateResolver.

Closes gh-3575

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

@@ -194,6 +194,7 @@ content into your application; rather pick only the properties that you need.
 	spring.thymeleaf.encoding=UTF-8
 	spring.thymeleaf.content-type=text/html # ;charset=<encoding> is added
 	spring.thymeleaf.cache=true # set to false for hot refresh
+	spring.thymeleaf.template-resolver-order= #  order of the template resolver in the chain
 
 	# FREEMARKER ({sc-spring-boot-autoconfigure}/freemarker/FreeMarkerAutoConfiguration.{sc-ext}[FreeMarkerAutoConfiguration])
 	spring.freemarker.allow-request-override=false