Issue Stats Without Assignee

### Problem Statement

Currently issue stats require a team filter, and the results only include issues assigned to that team or to people in that team. Our reality is that most issues are unassigned, and only occasionally get assigned for someone to look at. Our primary motive for using stats is to monitor noise / excessive issue creation, which does not align well with expecting all of these noisy issues to have an assignee. I made the mistake of automating the assignment of all new and previous issues to our primary team, and as a result I blasted each person in our organization with thousands of emails, because everyone in that team became implicitly subscribed to update notifications for every issue. My workaround has been to create a new team called `#sentry-analytics`, which is has no members and acts as the default assignee for all issues. This is just a hack so we can actually leverage issue stats, which is very useful to us.

### Solution Brainstorm

I would hope to see issue stats support a project multi-select filter and not require a team filter. Or alternately, make the team filter work by including all projects within that team, rather than via issue assignment. The current approach to the team filter is too tightly coupled to issue assignment, which is tightly coupled with issue subscriptions/notifications.

### Product Area

Stats

Start with the Stats area and review how the existing team filter selects issues. Compare the proposed project multi-select and team-by-project behavior, then define which filtering behavior should be supported and verify that unassigned issues are included without causing unwanted subscriptions or notifications.

Written by the indexing model from the issue text.