[Feature flag] Enable Service Level Agreements for Incident issues

What

Remove the :incident_sla_dev feature flag.

Owners

Team: devopsmonitor ~"group::health"
Most appropriate slack channel to reach out to: #g_monitor_health
Best individual to reach out to: @tristan.read or @seanarnold

Expectations

What are we expecting to happen?

This flag is used in the frontend and api to enable/disable this feature. It affects the operations settings page and the incident issues page, for incidents that have a linked alert.

What might happen if this goes wrong?

What can we monitor to detect problems with this?

Beta groups/projects

If applicable, any groups/projects that are happy to have this feature turned on early. Some organizations may wish to test big changes they are interested in with a small subset of users ahead of time for example.

gitlab-org/gitlab project
gitlab-org/gitlab-com groups
...

Roll Out Steps

Enable on staging (/chatops run feature set feature_name true --staging)
Test on staging
Ensure that documentation has been updated - !44879 (merged) and https://docs.gitlab.com/ee/operations/incident_management/incidents.html#service-level-agreement-countdown-timer
Enable on GitLab.com for individual groups/projects listed above and verify behaviour (/chatops run feature set --project=gitlab-org/gitlab feature_name true)
[-] Coordinate a time to enable the flag with #production and #g_delivery on slack.
[-] Announce on the issue an estimated time this will be enabled on GitLab.com
Enable on GitLab.com by running chatops command in #production (/chatops run feature set feature_name true)
Cross post chatops Slack command to #support_gitlab-com (more guidance when this is necessary in the dev docs) and in your team channel
Announce on the issue that the flag has been enabled
Remove feature flag and add changelog entry - !45085 (merged)
After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel

Edited Oct 18, 2020 by Sean Arnold