Cleanup Countdown manual construction of monograms/uris

Summary: looked for places where Countdown monograms/uris were being constructed by hand, and updated with modern versions

Test Plan: clicked around the Countdown UI, looking for broken links

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: chad, Korvin

Maniphest Tasks: T12524

Differential Revision: https://secure.phabricator.com/D17665