Add "en" LanguagePack on server startup, use cache of installed language choices

Does a few things:

1. On startup we check if english is in the language packs, then install if not
2. installed_language_packs endpoint now uses the `request.session['language_choices']` cache
3. add a new endpoint for forcing a refresh of the cache `refresh_installed_language_packs`
4. add some code in `update_languages` to invalidate the cache.