Make columns of user and ordergroup lists sortable

This commit implements the sort functionality for the user lists (by name, email, last_activity) and ordergroup lists (by name).
It is a first attempt addressing issue #560.

app/controllers/foodcoop/ordergroups_controller.rb

@@ -1,6 +1,6 @@
 class Foodcoop::OrdergroupsController < ApplicationController
   def index
-    @ordergroups = Ordergroup.undeleted.order('name')
+    @ordergroups = Ordergroup.undeleted.sort_by_param(params["sort"])
 
     unless params[:name].blank? # Search by name
       @ordergroups = @ordergroups.where('name LIKE ?', "%#{params[:name]}%")