Better alerting for client Sentry reports

The v1.14 client release was prompted by two bugs in the client which only got noticed once Sentry was deluged with so many reports that Sentry sent Nick an "Events quota exceeded" email. We should have noticed much easier.

While reacting to and manually triaging every new Sentry notification is difficult for the client for a variety of reasons, we should be able to sort out a form of alerting that does work for us - perhaps based on an uptick in the number of reports in a given time period, or if the number of users or events for a particular report exceeds some threshold.

Start by reviewing the current Sentry reporting and notification setup, then compare alerting approaches based on report volume, user counts, and event thresholds. Done means the team has an agreed alerting approach that would surface client incidents before Sentry sends an events-quota warning.

Written by the indexing model from the issue text.