Merges orders_helper functionality inside has_orders concern

Both these objects where using/setting a variable called valid_orders.
As a results, some specs where becoming flacky
(features/budgets/investments_spec.rb, balloting phase). By putting
them together, there is no `valid_orders` variable any more (everything
is @valid_orders) and the flackyness is gone.

app/views/shared/_order_selector.html.erb

@@ -4,7 +4,7 @@
           data-order="<%= @current_order %>"
           name="order-selector"
           id="order-selector-participation">
-     <% valid_orders.each do |order| %>
+     <% @valid_orders.each do |order| %>
        <option <%= 'selected' if order == @current_order %>
           value='<%= current_path_with_query_params(order: order, page: 1) %>'>
          <%= t("#{i18n_namespace}.orders.#{order}") %>