A/B test to beacon JS errors to Sentry

As discussed in the engineering meeting, I have built a [Sentry](https://getsentry.com/welcome/) service in EC2 for us to use for a week to see whether it is a good fit for our needs.

This PR uses the A/B framework to init the [Raven.js](https://github.com/getsentry/raven-js)  client for 20% of our users, essentially a cheap version of rate limiting (Sentry does support proper rate limiting, but I would need to scale the service to handle the load). 

### Reasons
- The flogger service is good but isn't really designed for errors:
- No visibility of UA
- No Stack trace
- No alerting

### Sentry supports:
- Rate limiting
- Error resolution/acknowledgement
- Ignoring/whitelisting of certain error types
- Filtering by user defined tags
- Notification/email system
- Source maps!
- Teams, users etc
- API

I will report back in a week to tell you how the test went. Please let me know if you want user credentials to log into the web interface.