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