Add HTTP proxy option for subscriptions

api/feed.go

api/payload.go

api/subscription.go

config/options.go

config/parser.go

database/migration.go

database/sql.go

database/sql/schema_version_35.sql

http/client/client.go

locale/translations.go

locale/translations/de_DE.json

locale/translations/en_US.json

locale/translations/es_ES.json

locale/translations/fr_FR.json

locale/translations/it_IT.json

locale/translations/ja_JP.json

locale/translations/nl_NL.json

locale/translations/pl_PL.json

locale/translations/pt_BR.json

locale/translations/ru_RU.json