Use local variables at shared/filter_subnav partial

So we can use it from components too.

app/controllers/concerns/has_filters.rb

@@ -1,5 +1,10 @@
 module HasFilters
   extend ActiveSupport::Concern
+  attr_reader :valid_filters, :current_filter
+
+  included do
+    helper_method :valid_filters, :current_filter
+  end
 
   class_methods do
     def has_filters(valid_filters, *args)

app/views/shared/_filter_subnav.html.erb

@@ -1,8 +1,8 @@
 <ul class="menu simple clear" id="filter-subnav">
   <li class="show-for-sr"><%= t("#{i18n_namespace}.filter") %>: </li>
 
-  <% @valid_filters.each do |filter| %>
-    <% if @current_filter == filter %>
+  <% valid_filters.each do |filter| %>
+    <% if current_filter == filter %>
       <li class="is-active"><h2><%= t("#{i18n_namespace}.filters.#{filter}") %></h2></li>
     <% else %>
       <li><%= link_to t("#{i18n_namespace}.filters.#{filter}"),