Need a separate field in repos.yml for notifications Slack channels

[sengi]

The team field (which is supposed to be the team's Slack channel as listed on the Application page and the Repo page) is also being used for sending automated notifications to teams' Slack channels.

These two usages are not compatible, because some teams don't want automated messages to go to their main channel and have separate channels for robots/notifications.

If other tools are to continue using repos.yml for their configuration then we need a separate field for specifying the Slack channel to use for notifications. (I'd suggest slack_notifications_to.)