Allow virtual users to be members of multiple teams with different effective dates

Development is largely a shared resource and it is very common for development teams to be re-balanced at the end of each program increment (PI). Specifically, developers will often be re-assigned from their current team over to a new team for the next PI.

We need a way to factor this into our capacity plans in Aha! Today, I can assign a user to multiple teams however there isn't a way to indicate when those users will be active on those teams. We need a way to specify a date range for per virtual user per team, and the Capacity Report should factor this into the overall team capacity.

We can workaround this today by defining entirely new teams, however that leads to significant overhead particularly when the team membership is mostly staying the same.