Crashlytics

As per the [discussion in the forum](https://groups.google.com/forum/#!topic/anki-android/RQqxcITIqYE), I want to try out Crashlytics as a replacement for the current Triage. With this PR we will have access to the logcat together with the stacktrace, and will be able to search for reports sent by a given user given their UUID which they can find in the About screen.

Because our current Triage becomes completely non-functional within a few weeks of release day, I want to push this into 2.4 so that we can fix some bugs. Currently the only limitation is that there is no way for the user to disable automatic sending of crash info. I've contacted the crashlytics team to find out how to do that, but for now I think we can live with it.