Move admin/pubsubhubbub controller to admin/subscriptions (#3442)

app/controllers/admin/pubsubhubbub_controller.rb

app/controllers/admin/subscriptions_controller.rb

app/views/admin/pubsubhubbub/index.html.haml

app/views/admin/subscriptions/_subscription.html.haml

app/views/admin/subscriptions/index.html.haml

config/locales/ca.yml

config/locales/de.yml

config/locales/en.yml

config/locales/fa.yml

config/locales/fr.yml

config/locales/he.yml

config/locales/id.yml

config/locales/io.yml

config/locales/ja.yml

config/locales/no.yml

config/locales/oc.yml

config/locales/pl.yml

config/locales/pt-BR.yml

config/locales/pt.yml

config/locales/ru.yml