More work on new search UI (follow-up to r50207):

More work on new search UI (follow-up to r50207):
* introduce search.js (loaded only for Special:Search):
** updates target links when search term is changed and one of the header links clicked
** added Aude's code for toggling advanced search checkboxes
* fix minor styling issues in IE

Tried different ways to make prefix: queries look smarter, but will leave it alone for now, since there is
no neat way of fixing them.