Clean up permission updater tests.

Be consistent about how we assert on update calls.  Was previously using
a mixture of webmock, and mocha mocks.  Also remove some unnecessary
detail in some of the stubs.
Improve robustness of permission update worker

Gracefully handle:

* The user no longer existing
* The application no longer existing
* The user no longer having any permissions for the application

Any of these can change between when the job is scheduled, and when it's
actually run.
Improve robustness of the reauth enforcer worker.

It's possible for the application to have been deleted between when the
job was scheduled, and when it's actually run.
Merge pull request #238 from alphagov/update_worker_robustness

Improve robustness of user update workers