Closes #2237 Depends on engine!402

Summary

Adding in an option for admins to retranscode a video manually.

Testing

1. Log in as admin (ask if you don't have the password) - refresh.
2. Get a video, or post a video
3. Check the dropdown menu
4. You should see retranscode as an option
5. Open your browser console on the XHR/Requests tab
6. Click it, check v2/admin/transcode is called and returns success.

1. Non admins and not logged in should NOT see the retranscode option.

note Transcoding on sandboxes may not work, you may need to manually run the transcoder queue if its not up.

Using kubectl to gain access to the sandbox container, enter the engine dir, and run php ./cli.php QueueRunner run --runner=Transcoder

added scoped labels

changed the description

mentioned in merge request engine!402