Fix site header search
Fix site header search

The search route has been updated to require the 'get' method
in https://github.com/mysociety/alaveteli/commit/416ad02a

Using the 'post' method doesn't work anymore.

This commit updates the request method in our theme’s site header
search form.

Fixes #599

openaustralia/righttoknow,openaustralia/righttoknow,openaustralia/righttoknow