Make User Management more fine grained

### Problem Statement

Currently I only see an option, to make each configured team open to join or each configured team locked and only joinable by approval.
It would be great, to have the option, to either make this Open Membership configureable for each single team or even configure a "default" team in the same way a default role is configureable.
We would like to have Project specific teams, that we want to use for alerting and make them only joinable by approval.
Further new Users should be automatically assigned to a default team, so that those new users are able to see every integrated project in Sentry, without the need of additional steps.

### Solution Brainstorm

- make Open Membership configurable for each team
- add the option for a default team for new users

### Product Area

Settings - General