Make "reviewer" a tag instead of a status

[LeMurphant]

Follows from https://github.com/StampyAI/stampy-ui/issues/762

This must be done in order-ish

• [x] Create the "In-review" internal tag (me)
• [ ] Make "@reviewer" add the "In-review" tag (@mruwnik )
• [ ] Create a Coda page to view the tag (@plexish )
• [ ] Move the articles status from "In review" to "Unlisted" or "in-progress" after tagging them (me)
• [ ] Remove "@reviewer" status change
• [ ] Remove the "In review" status