Add a `$__teams` global variable

Discussed in https://github.com/grafana/grafana/discussions/38196 and now referring to https://github.com/grafana/grafana/issues/82940 as well. Re-Opening because the Issue is very good in my use case. Originally posted by ethirolle December 28, 2020 **What would you like to be added:** Add a global variable ($__teams) that contains a list of the teams that the current user is a member of. **Why is this needed:** This would be similar to the $__org global variable: https://grafana.com/docs/grafana/latest/variables/variable-types/global-variables/#__org. It would allow the Grafana admins to customize a dashboard to return different results (or use different datasources) based on team membership. In the customer's words: > Scenario: If we have per-team datasources and want to have a shared dashboard where the datasource points to that team's datasource automatically. > The more generic way might be to be able to define custom template variables for a team > So the template variable could be used as the datasource name or part of the mane An alternative fix for this same problem is described in this issue, which asks for per-team variables: https://github.com/grafana/grafana/issues/21071