i18n: full Korean translation. follow up the changes up to this commit.

Now we have a fully translated Korean version of Discourse. 
(100% complete up to now)
app/views/static/faq.ko.html.erb
app/views/static/privacy.ko.html.erb
app/views/static/tos.ko.html.erb
config/locales/client.ko.yml
config/locales/server.ko.yml
public/403.ko.html
public/422.ko.html
public/500.ko.html
public/503.ko.html