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 
After thinking about it, I believe this is vulnerable. If an attacker could modify the html contents of the element selected on line 213, then he could execute arbitrary code on line 214. I'm not sure how an attacker could exploit this in practice, but I think I will try to escape the output on this line and line 216 to be safe.