Change admin language pack selection dropdown

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Change the language pack download selection dropdown in `/update/languages` so
that it uses the more arbitrarily stylable Bootstrap components, instead of
`select` and `option` elements. See:
http://getbootstrap.com/components/#dropdowns
Also improved readability of some JS by giving more sensible function names and
through judicious use of jquery instead of `sprintf`ing html elements.
Included a placeholder image to signify beta language packs.

Solves #2720 
This particular code isn't equipped for it yet, so this is just for context: we would now normally handle this with Handlebars templates -- makes the HTML really clean to read (similar to Django templates), while still being client-side renderable and also making sure to prevent XSS.