Replace the legacy raven-js package with Sentry's new JS SDK [1], with a
minimal wrapper in `util/sentry.js` that exports the same interface as
the previous `raven.js` module.

To enable Sentry, h or the browser extension must set a `sentry` key in the
sidebar app's configuration.

As part of switching SDKs, the following changes were made:

 - The AngularJS integration was removed, as we are in the process of
   migrating away from Angular
 - The custom code for unhandled promise rejections was removed, as the
   new SDK handles this already
 - The URL rewriting logic was removed. This was done to make the setup
   as minimal as possible. This may need to be re-added later.
 - The `report` function was removed, as it was not used anywhere
 - The `session` module now imports `util/sentry` directly rather than
   using Angular dependency injection. Now that we have a convenient way
   to mock all CJS/ES imports, there isn't a need to use DI for
   utilities

[1] https://docs.sentry.io/platforms/javascript/