Allow custom proxy host header to be set (#393)

* ✨ feat: proxy header override

* 🐛 fix: missing translations key

* 🐛 fix: missing translations key, again 😅



* Update src/nginxconfig/i18n/en/templates/domain_sections/reverse_proxy.js

Co-authored-by: Matt Cowley <me@mattcowley.co.uk>

* 💡 chore: bump copyright year

* 🎨 refactor: update key to proxyHostHeader and i18n messages

* 🎨

 refactor: fix key in template

Co-authored-by: Matt Cowley <me@mattcowley.co.uk>