Improve ruleset for generating project hashtags

Summary:
Ref T9551. We currently use the same logic for generating project hashtags and Phriction slugs, but should be a little more conservative with project hashtags.

Stop them from generating with stuff that won't parse in a "Reviewers:" field or generally in commments (commas, colons, etc).

Test Plan:
Created a bunch of projects with nonsense in them and saw them generate pretty reasonable hashtags.

{F873456}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9551

Differential Revision: https://secure.phabricator.com/D14261