Make the search form a GET form with no extra fields

We only accept GET requests for the search, and although the form is
never normally submitted directly some browsers offer an option to
create a bookmark from a form, so make that work.

app/views/layouts/_search.html.erb

@@ -1,7 +1,7 @@
-<%= form_tag search_path, :class => "search_form" do %>
+<form method="GET" action="<%= search_path %>" class="search_form">
   <%= submit_tag t('site.search.submit_text') %>
   <div class='query_wrapper'>
     <%= text_field_tag "query", params[:query], :placeholder => t("site.search.search"), :autofocus => autofocus %>
     <%= link_to t('site.search.where_am_i'), '#', { :class => "describe_location", :title => t('site.search.where_am_i_title') } %>
   </div>
-<% end %>
+</form>