Fixed loading pt-br translations

fixes https://github.com/TryGhost/Ghost/issues/16805

* turns out i18next requires the country portal of the locale to be
  capitalized [1]
* this resolves that and moves the files to the correct casing

[1]: https://github.com/i18next/i18next/issues/1467#issuecomment-646305827

ghost/i18n/lib/i18n.js

@@ -19,7 +19,7 @@ const SUPPORTED_LOCALES = [
     'no',
     'pl',
     'pt',
-    'pt-br',
+    'pt-BR',
     'ru',
     'si',
     'sl',