Renamed template files' extension to .mustache

Using .java and .scala as file extensions for the template files messes up IDE's - they try to parse the files as valid java/scala code, and show a lot of red errors.

I also had problems with running actual Scala code, since the template files having a .java or .scala extension were scheduled for compilation, which failed.

Some IDE's (like IntelliJ) have Mustache syntax highlighting, so it makes more sense to highlight the templates as Mustache code, treating it as Scala embedded in Mustache, not Mustache embedded in Scala.