Simplify some logic in project controllers

Summary: Ref T10010. Several controlers currently have similar logic for handling tags and slugs, loading projects, and canonicalizing URIs. Clean it up a bit.

Test Plan:

• Visited profile, boards, feed.
• Visited by ID and by tag.
• Visited by non-normal tag (redircted).
• Visited by alternate tag (redirected).
• Visited non-policy project by non-normal tag (redirected into policy error).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D14890