Add mixpanel ping for next browser release

[drshore]

Do we have any way of knowing how many people are using Realm Browser?

@astigsen and I were discussing that we should have a way of understanding how many browser users we have. This is late in the process, but for the last release of browser where we add the notification of deprecation, we should have a ping so we have a rough estimate of browser usage, and be able to translate that to how many people are switching to studio.