Guidelines for support personell raising incidents for GitLab.com

Our incident-management framework currently does not describe the case where support team members are dealing with a number of reports of an issue from users, but no alerts have been raised via our monitoring system.

One example of this is here: https://gitlab.slack.com/archives/CB7P5CJS1/p1560188110002400.

As GitLab.com becomes adopted by more organisations and paid users, we will need to ensure a proper escalation path for these reports.

We should consider updating the incident management framework to include guidelines for when the support teams should manually raise an incident ticket and/or pagerduty page to notify the on-call of a suspected problem.

cc @glopezfernandez @ansdval @Finotto @dawsmith @tatkins @lbot @lyle