Generate Phriction URI from saved Project slug, not project name

Summary: We recently changed the slug normalization rules, so project names may now generate a different slug than they used to. We have the correct slug saved, though, so just use that.

Test Plan: Created some projects with various names full of garbage and verified they link correctly.

Reviewers: AnhNhan

Reviewed By: AnhNhan

CC: mbishopim3, aran

Maniphest Tasks: T2632

Differential Revision: https://secure.phabricator.com/D5263