Stats for Number of Alerts Created

### Problem Statement

As an admin of a large Sentry org, it would be nice to get a breakdown in the Sentry UI of where issue and metric alerts have been created, especially given the current limits on the number of alerts that can be created org-wide.

### Solution Brainstorm

- Add a visual method in Stats view to quickly check per-project alert totals
- Easier: add stats to the bottom of the Alert search that show "x out of y" where x is the number of alerts on the current page, and y is the total number of alerts found by the search

### Product Area

Alerts