_config.yml: english page is now built with _config.en.yml

Fix include and exclude config in order to build English page (such as
javascript/application/ieerbug/index_en.md and
javascript/application/racing/2_en.md) with _config.en.yml.
add _includes/set-locale.html and stop using locale_filter plugin
delete locale_filter plugin
Delete locale_filter plugin and add _includes/set-locale.html