Fix TeamDetailsEndpoint.delete() to properly defer deletion

The countdown argument is used with Task.apply_async(). When calling
it on Task.delay it's just passed through direectly to delete_team()
where it is never used.
Make TeamManager.get_for_user() ignore non-visible teams

Teams with status != TeamStatus.VISIBLE are pending deletion.
Hide them from normal view.
Convert remove_team() to use async delete_team()

Previously remove_team() called team.delete() directly, which
orphans the team's projects and other related objects. Meanwhile,
TeamDetailsEndpoint.delete() uses the asynchronous delete_team() task,
which nicely handles related object cleanup.
Merge pull request #1315 from jaysoffian/async_ui_team_delete

Async ui team delete