I18n refactoring

This depends on #1736.

Not so much refactoring as cleaning...
* Remove `LanguagePack` model (whose presence recently caused some confusion)
* Remove commands and functions exclusively for central server creation of language packs
* Move code for language pack download into `languagepackdownload`

Next steps of refactor must be taken on the central server side, including:
* Defining apps on the central server side that preserve this segregation
* Publishing public API definitions and resource URIs.