Sentry error tracking

There are two components of using Sentry:

1. We've added several lines of code to the beiwe-backend repo (the server code) to call functions that send stack traces to a Sentry instance. The Android app doesn't have any code in it that calls Sentry functions; the Android app uploads stack traces to the backend, and the backend forwards those stack traces to Sentry. The few lines of code we've added to the beiwe-backend repo should be fine to open-source; they're similar to the lines of code that call AWS functions, in that they hook into services outside Beiwe.

2. The Sentry instance itself is entirely separate from the Beiwe code. There are two options for running Sentry: the Sentry code is available under the BSD open source license for people to install themselves, or you can pay Sentry to configure and host it for you. In our case, I think paying Sentry $12/month for hosting would be cheaper than paying AWS for hosting and paying us for engineering time to do setup and maintenance costs. Any future researchers who use Beiwe open source code would have three choices: a.) not use Sentry, b.) pay Sentry for a hosted instance, c.) host their own instance of Sentry.