Use monograms for Herald URIs

Summary: I think HeraldRules are the only objects which have monograms but are not accesible via /{$monogram}. This diff changes the /herald/rule/{$id} URI to /{$monogram}.

Test Plan: Clicked a bunch of links in Herald to ensure there were no dead links.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14469