Adds issue template for feature flag roll out

.gitlab/issue_templates/Feature Flag Roll Out.md

+<!-- Title suggestion: [Feature flag] Enable description of feature -->
+
+## What
+
+Remove the `:feature_name` feature flag ...
+
+## Owners
+
+- Team: NAME_OF_TEAM
+- Most appropriate slack channel to reach out to: `#g_TEAM_NAME`
+- Best individual to reach out to: NAME
+
+## Expectations
+
+### What are we expecting to happen?
+
+### What might happen if this goes wrong?
+
+### What can we monitor to detect problems with this?
+
+<!-- Which dashboards from https://dashboards.gitlab.net are most relevant? -->
+
+## 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-ce`/`gitlab-org/gitlab-ee` projects
+- `gitlab-org`/`gitlab-com` groups
+- ...
+
+## Roll Out Steps
+
+- [ ] Enable on staging
+- [ ] Test on staging
+- [ ] Ensure that documentation has been updated
+- [ ] Enable on GitLab.com for individual groups/projects listed above and verify behaviour
+- [ ] Announce on the issue an estimated time this will be enabled on GitLab.com
+- [ ] Enable on GitLab.com by running chatops command in `#production`
+- [ ] Cross post chatops slack command to `#support_gitlab-com` and in your team channel
+- [ ] Announce on the issue that the flag has been enabled
+- [ ] Remove feature flag and add changelog entry
+
+/label ~"feature flag"