Make normalization of "#yolo" hashtags less aggressive

Summary: Fixes T3825. See that task for details.

Test Plan: Verified that `#\herp` no longer matches project `#herp`, but `#herp` still works fine.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3825

Differential Revision: https://secure.phabricator.com/D6970