Issue Alerts - If filters which apply to the issue group, not just the event

### Problem Statement

Example alert: https://dropbox.sentry.io/alerts/rules/dropbox-android/15040727/

We have many alerts at Dropbox and one of the constraints we regularly hit is that the `IF` filters only apply to the individual event itself, not the group.

In the alert above, this is an issue because this exception is sometimes caught and sometimes results in a crash (level is usually `error` but occasionally `fatal`). When a `fatal` level event comes in, it checks for the >6000 users/wk count constraint, in the `WHEN` section, which _does_ check against the issue group. However, the `IF` section does not filter the group for events matching these `IF` constraints. This is not obvious, nor intuitive when configuring the alerts.

### Solution Brainstorm

Ideally we'd like to be able to filter the count of users to only `fatal` events in the group. In more generic terms, we'd like for the `IF` terms could be used as filter's on the issue group before evaluating the `WHEN` clauses. We recognize that this has performance implications, but nonetheless it is probably our ideal solution.

Secondarily, at minimum, Sentry should tweak the UI to make the behavior more obvious.

### Product Area

Unknown

Start with the linked Dropbox alert and compare how the IF filters and WHEN count constraint behave for events in the same issue group. Read the alert behavior described in the issue, then determine whether the intended outcome is group-level filtering, clearer UI behavior, or both; done means the selected behavior is explicit and validated against this example.

Written by the indexing model from the issue text.