Close and reopen epic via quick action

Problem and proposal

• For GitLab to use this, we need everyone (all GitLabbers that is) to be able to close and open/reopen epics. But we don't everyone deleting them. So we have to create the ability to open/reopen and close.
• This issue is the first step. We chose to use a quick action as a first step to minimize scope in this small iteration (Summit). In future iterations, we will add other standard features like closing via buttons in the UI. See &145 (closed).

Requirements

• Reporter+ role in the group can close an epic.
• Reporter + role in the group can reopen an epic.
• Epic view of a closed epic. Probably an open badge and a closed badge.
• Buttons to close/reopen. https://gitlab.com/gitlab-org/gitlab-ee/issues/7013.
• System note. https://gitlab.com/gitlab-org/gitlab-ee/issues/7015
• Quick actions to close an epic and reopen an epic.
  • /close Close this epic
  • /reopen Reopen this epic
• Comment and close button. https://gitlab.com/gitlab-org/gitlab-ee/issues/7013
• Tabs on the epics list to show open versus closed. https://gitlab.com/gitlab-org/gitlab-ee/issues/7014
• API updates to expose the epic states and changing the epic states of open vs closed.
• When this feature is first implemented, epics should be open by default. (Not sure if a DB migration is necessary to support this.)