Add row of available languages for /type/i18n_page

We make use of the `/type/i18n_page` for a few things now: eg https://openlibrary.org/subjects , https://openlibrary.org/help/faq/editing , etc. Editors should be aware that pages are i18n-pages, so as to help keep them in sync.

### Proposal & Constraints
A bar at the top of an i18n page that lists the languages; eg:

> This page is available in: [de](https://openlibrary.org/subjects?lang=de) | [**en**](https://openlibrary.org/subjects?lang=en) | [es](https://openlibrary.org/subjects?lang=es) | [fr](https://openlibrary.org/subjects?lang=fr)

(current language being bold)

### Additional context

Related files:

- i18n edit page template: https://github.com/internetarchive/openlibrary/blob/master/openlibrary/templates/type/i18n_page/edit.html
- i18n view page template: https://github.com/internetarchive/openlibrary/blob/master/openlibrary/templates/type/i18n_page/view.html

### Stakeholders
@dcapillae