BURs: add processing and failed states.

When a BUR is approved, put it in a `processing` state. After it
successfully finishes processing, put it in the `approved` state. If it
fails processing, put it in the `failed` state.

If approving the BUR fails with a validation error, for example because
the alias already exists or an implication lacks a wiki, then leave the
BUR in the `pending` state. The `failed` state is only for unexpected
errors during processing.

test/functional/bulk_update_requests_controller_test.rb

@@ -181,7 +181,10 @@ class BulkUpdateRequestsControllerTest < ActionDispatch::IntegrationTest
       context "for an admin" do
         should "succeed" do
           post_auth approve_bulk_update_request_path(@bulk_update_request), @admin
+
           assert_response :redirect
+          assert_equal("processing", @bulk_update_request.reload.status)
+          perform_enqueued_jobs
           assert_equal("approved", @bulk_update_request.reload.status)
         end
       end