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 
Here's some info about how we use HB: https://github.com/learningequality/ka-lite/wiki/Approach-to-client-side-templating-(Handlebars.js) (so far, we've only been using it in our new code, from withing Backbone, but it could just as easily be used outside Backbone).

If the langdata were controllable by a 3rd party (e.g. loaded from a DB, or from a GET param), then line 160 (passing it to `.html()`) could lead to script injection. The only way that would happen here is if the requested AJAX JSON were MITMed (on its way from the central server) and had JS injected. It should now be via HTTPS, so that shouldn't be possible, but otherwise it theoretically could.