Add support for overriding strings in the app (#7886)

* Add support for overriding strings in the app

This is to support a case where certain details of the app need to be slightly different and don't necessarily warrant a complete fork. 

Intended for language-controlled deployments, operators can specify a JSON file with their custom translations that override the in-app/community-supplied ones.

* Fix import grouping

* Add a language handler test

* Appease the linter

* Add comment for why a weird class exists

src/SdkConfig.ts

@@ -29,6 +29,8 @@ export interface ConfigOptions {
     // sso_immediate_redirect is deprecated in favour of sso_redirect_options.immediate
     sso_immediate_redirect?: boolean;
     sso_redirect_options?: ISsoRedirectOptions;
+
+    custom_translations_url?: string;
 }
 /* eslint-enable camelcase*/