search: search form as files

* Splitting the `search_form` macro into files. All the blocks have been
  preserved.

* Deleting the `search_form` macro.

* NOTE if you were using the `search_form` macro you should replace it
  by `{% include "search/form/index.html %}`

Signed-off-by: Yoan Blanc <yoan.blanc@cern.ch>