Remarkup links to link to short url instead of long and fix commenting on Phurl's

Summary: Ref T6049, remarkup links to use short URLs and make commenting on Phurl's actually work

Test Plan:

• Create Phurl U123
• Comment on that Phurl ((123))

Comment should link to /u/123

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T6049

Differential Revision: https://secure.phabricator.com/D14477