Allow users to disconnect teams

Right now if we want to disconnect teams from Scoreboard, we need to manually delete the entry for a user's access token from the Scoreboard database. We should have a piece of UI to allow a user to disconnect their team, or for admins to delete these entries.

Start by tracing the existing team connection and access-token flows in Scoreboard, then identify where the user-facing and admin controls belong. Done means users can disconnect their team, or administrators can remove the associated access-token entry, without manual database changes.

Written by the indexing model from the issue text.