add external_service parameter to thread API

app/api/route/thread_api.rb

@@ -7,6 +7,7 @@ class ThreadApi < Base
       optional :issue_id, type: Integer, desc: 'ID of issue'
       optional :order_by, type: Symbol, values:%i( created_at id  ), desc: 'Order of returned issues.'
       optional :order, type: Symbol, values:%i(asc desc), default: :asc
+      optional :external_service, type: String, desc: 'Filter by external service short name'
     end
 
     helpers do
@@ -19,6 +20,7 @@ def post_or_get_thread
         end
         scope = scope.order(params[:order_by] => params[:order]) if params[:order_by]
         scope = scope.where(issue_id: params[:issue_id]) if params[:issue_id]
+        scope = scope.joins(:external_service).where('external_services.short_name' => params[:external_service]) if params[:external_service]
         scope = paginate scope
       end
     end