Move issue/epic/incident sidebar items into actions dropdown menu

Background

We still have the MR sidebar updates behind a feature flag but we'd like to enable it globally. In order to do so, we should probably ensure that there's a little bit more consistency between issues, epics, (other issuables?), and merge requests.

Proposal

• Move several sidebar items into the dropdown at the top to maintain relative parity with MR - lock, copy reference, copy email, notifications
• Add popover to alert users to the moved items
  • Popover should appear on both issues and MRs
  • Once dismissed from either location, it shouldn't show again
  • Popover copy: Notifications, <issue/merge request/etc> lock, and other actions are available in this dropdown list.
• Add behind same :moved_mr_sidebar feature flag

Issue

• Notifications
• Lock issue
• Copy reference
• Copy email email

Epic

• Notifications
• Copy reference

Incident

• Notifications
• Copy reference
• Copy email

Current	Proposal	Popover