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 
Yeah, similar to the other case (if they could MITM the JSON being requested from the central server, containing the language data, which populates the options list). Since that's over HTTPS, though, it's probably low risk.

However, we don't want to have to rely on our ability to think through all the ways user-generated (or injected) content could end up being passed to `.html()`. When we use something like Handlebars, that escapes all strings it renders, and then have a rule that the *only* thing we're allowed to pass to `.html()` is the the output of rendering a Handlebars template, we're safe (on the XSS front) by default.