Ability to set a default team for new members to be added to

### Problem Statement

See https://github.com/getsentry/sentry/issues/44013 for first request

Use case - users that are added to the organization but not to any teams cannot be "Assigned To" on issues. Having a default team fixes this issue.

Alternatively, if you could just make it so that teamless members can be assigned to for issues, that would also suit my purposes :)

### Solution Brainstorm

_No response_

### Product Area

Settings - Teams

Start by reviewing the related request in getsentry/sentry#44013 and the Settings - Teams area. No files, tests, or entry points are named; done would require a decided approach for default-team assignment or assigning teamless members, with the behavior covered by appropriate tests.

Written by the indexing model from the issue text.