Improved UX when adding a team member in the Developer Dashboard

Previously we could re-add a team member using the UI from the Settings > Team page (or just the Team page from the Beta Developer Dashboard).

We discovered this edge case bug and fixed it with a notification that the person is already a team member.

This also prevents accidentally using our API to demote oneself from Owner or Admin. In order to change the permission of a team member, please use the dropdown menu next to the member.