Change admin language pack selection dropdown

-----
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 
No, the choice of an image was arbitrary. It would be really simple to add styled text instead, just by changing a line or two of javascript and adding some styling to the appropriate stylesheet.