Admin: add support for updating languages

[monfresh]

There are too many possible languages to have them listed as checkboxes. A multi-select dropdown could work, but would probably be too cumbersome. Autocomplete/typeahead would probably be the best solution, and I think the solution covered in this Railscasts looks perfect to me:

http://railscasts.com/episodes/258-token-fields-revised?view=asciicast

The Railscasts uses Chosen as the jQuery plugin, but also mentions Select2. I'll read about both to see which would be a better fit.

@anselmbradford would love your thoughts. Have you used either of those plugins?

[anselmbradford]

I haven't used them, but have looked at Select2 before. There's this one also: https://github.com/twitter/typeahead.js