langpicker: Various UI fixes

See bug #3181
js: Update select2 to 3.4.5

One of the things we need most of all is diacritics support, for the
language picker.

See: https://github.com/ivaynberg/select2/issues/1472
breadcrumbs: Remove the style="visibility: hidden" from select2 menus

The select2 update takes care of doing this for us; if we leave it in
the initial style the select2 menu will copy it and the menu disappears.