Clear search alerts when a forum topic is selected

common/static/coffee/src/discussion/views/discussion_thread_list_view.coffee

@@ -493,6 +493,7 @@ if Backbone?
     clearSearch: ->
       @$(".forum-nav-search-input").val("")
       @current_search = ""
+      @clearSearchAlerts()
 
     clearFilters: ->
      @$(".forum-nav-filter-main-control").val("all")