This commit adds auto-configuration and a starter,
spring-boot-starter-freemarker, for using FreeMarker view templates in
a web application.

A new abstraction, TemplateAvailabilityProvider, has been introduced.
This decouples ErrorMvcAutoConfiguration from the various view
technologies that Spring Boot now supports, allowing it to determine
when a custom error template is provided without knowing the details of
each view technology.

Closes #679