Add support for opting in an entire team

[lerebear]

At present, the optIns configuration only supports listing individual users. This makes it more cumbersome for an entire team to adopt the tool, because it first requires them to list each existing team member individually, and then to continually update the list of users when team membership changes.

To make this easier, we should support references to a team in the optIns config. The team name could be specified as it's GitHub handle without the leading @ symbol (i.e. /). We would also need to advertise that the team must have access to the repository for the config to take effect.