Require emails for slack oncall usergroups to match @gitlab.com
What
Updates the slack usergroup updater to skip any email that does not end with @gitlab.com
.
Why
We have a case of someone getting re-hired. They still have an alumni account in slack. Their PagerDuty profile includes the email used by said slack profile as a secondary email.
Updating the usergroups fails because their alumni account is a single-channel guest.
By enforcing the email to be a GitLab one, we avoid this problem.