search: search form as files

* Splitting the `search_form` macro into files. All the blocks have been preserved.
* Deleting the `search_form` macro.

The goal of this is to make the customization of the `search_form` easier. You should have to redo it to _extend_ it. See: https://github.com/inspirehep/inspire-next/blob/2e01dab478f1042dd7b72b1ae3f5eb9a77c71842/inspire/base/templates/search/helpers.html#L35-L199

Ping @giokokos, @jmartinm, @drjova for comments, review or test. Thanks mate!

I'm using it here: https://github.com/greut/cds-demosite/tree/cds-layout