R1:feca8fbdece3

Use monograms for Herald URIs

Summary: I think `HeraldRule`s 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