Fix matching of very short project hashtags ending in a digit

Summary: Fixes T7625. The way the regexp worked, "unusual" terminal characters required at least one character as a prefix in order to match. Allow 0 instead, so #a1 matches.

Test Plan: Added and executed unit test.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7625

Differential Revision: https://secure.phabricator.com/D12123