[Feature flag] Enable Design Management feature by default

What

Enable the :design_management_flag feature flag by default.

Owners

• Team: groupknowledge ~Create
• Most appropriate slack channel to reach out to: #g_create_knowledge
• Best individual to reach out to: @.luke

Expectations

### What are we expecting to happen?

The Designs tab of issues should become visible (unless LFS has been disabled for the Project).

What might happen if this goes wrong?

Not sure. One possibility is that Design Management does not scale in some area.

What can we monitor to detect problems with this?

• https://sentry.gitlab.net/gitlab/gitlabcom/

Beta groups/projects

• 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 (https://gitlab.com/gitlab-org/gitlab-ee/issues/13709)