Update jQuery to 1.4.0.

In order to get the Chosen jQuery plugin working.
Install the Chosen jQuery plugin.

Version is the current HEAD + a patch sent upstream [1].

[1] https://github.com/jiru/chosen/commit/cfcdc325f5cb5e474cca318d5e21094797e380df
Use Chosen for language selection dropdowns.

This will hopefully ease the selection of languages.
Adapt Chosen dropdowns CSS to fit Tatoeba’s style.
Fix Chosen dropdowns bug.

Chosen dropdowns are getting hidden once they get out of the
page contents’ div. This is especially annoying for dropdowns
that are on the bottom of the page, such as the ones in
/activities/translate_sentences. I’m not sure this change won’t
break other things but it sounds okay. For the record, this line
was introduced by commit b0b52ad6283f6be54786196c7ced3c4d64edf6a3.
#364 Fixed language select height in sentences/show
#364 Fixed padding/margin for language select in annexe content
Merge pull request #364 from Tatoeba/lang-selector

Better language selection dropdown