Slack channel data can get out of sync

[AgaDufrat]

Currently "channel” in the alphagov.yml config e.g. #govuk-publishing-platform-system-alerts has be kept in sync with “alerts_team” in the repos.json from dev-docs to be able to get a list of repos in the TeamBuilder class.

We should have a single source of slack channel for alerts in repos.json (under “alerts_team”) to avoid having to keep those two in sync.

We could base the repos search on team_name instead. team_name in the alphagov.yml config e.g govuk-publishing-platform has to match “team” in the repos.json. “alerts_team” falls back to "team" when it's not specified.

Note: GOV.UK may also want "quotes", used for team charters etc, sent to their "main" channel.

Why

This would reduce the risk of those 2 data sources getting out of sync and simplify the process of configuring alerts when team structure or names change.