Make "reviewer" a tag instead of a status

[LeMurphant]

Follows from #762

This must be done in order-ish

• Create the "In-review" internal tag (me)
• Make "@Reviewer" add the "In-review" tag (WIP, see Change @reviewer functionality from status to tag stampy#331)
• Create a Coda page to view the tag (this will do for now )
• Move the articles status from "In review" to "Unlisted" or "in-progress" after tagging them (me)
• Remove "@Reviewer" status change (WIP, see Change @reviewer functionality from status to tag stampy#331)
• Remove the "In review" status
• Cleanup Coda formulas that use "In review"