I was surprised when I tried to spread a `Set` in TypeScript and the browser complained about `Set.slice()` not being defined. This is because TypeScript does not automatically enable support for iterators when targeting earlier ES versions, like we do in the browser, unless you use the `"downlevelIteration": true` compiler option. This injects some helpers into the necessary files for reading/spreading iterators, which can be stuffed behind an import statement with using the `"importHelpers": true` compiler option and include `tslib` in our dependencies. This is already a dependency of several of our packages, so it shouldn't cause any additional modules.
* set kibana locale in kibana.yml config
* remove accept-language-parser
* remove unnecessary tests
* fix readme description, fix description for locale in kibana.yml
* add point, that i18n.locale option should have exact match
* update kbn/i18n README
* Update README.md
* use getUiTranslations in render_mixin, remove i18n_mixin
* move registering translation files to mixin function
