ILanguageManager should be exposed as a REST resource

Original LP Author: Sumana Harihareswara , LP Link: https://bugs.launchpad.net/bugs/1414298

Expose ILanguageManager as a REST resource, so that Postorius can use it to learn what languages a system supports, and thus provide list administrators with options for a list's preferred language. Refer to src/mailman/model/docs/languages.rst -- I imagine this will probably use a getUtility call to say,

getUtility(ILanguageManager)

Background, based on my conversation with Barry yesterday (I may have gotten some things wrong):

The system knows it supports certain languages. This is set in a config file that only the site administrator can change -- a change in the config file requires a mailman restart in order to take effect. The Language Manager is per system.

The Language Manager is a utility, like the user manager (that is, essentially a singleton in the system; interfaces that have multiple implementations are not utilities). When the system boots up, it registers implementations of interfaces. These are ZopeInterfaces, as we use the Zope component architecture, ZCA.