Normalize project slugs before querying for them

Summary:
Fixes T5728. In particular:

• /tag/XYZ/ now works as an alias for /tag/xyz/.
• arc todo --project ASDF now works as an alias for arc todo --project asdf.

Test Plan: Called project.query and visited /tag/LBHABLHBH/.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aklapper, epriestley

Maniphest Tasks: T5728

Differential Revision: https://secure.phabricator.com/D10144