This commit changes the default file extension for Mustache templates,
from `.html` to `.mustache`, which is the file extension used in the
official reference documentation and by most IDE plugins.
Fixes gh-8997
The package names changed a bit from the prototype project, but wuth vanilla
autconfiguration usage that shouldn't matter. Follows closely the Groovy
templates support. Templates live in classpath:/templates/*.html by default.
Fixes gh-2242