Markup project hashtags which begin with (or contain only) digits

Summary:
Fixes T9832. We currently refuse to recognize project hashtags in remarkup if they begin with a digit. This is motivated by attempting to not recognize them if they contain only digits.

I don't think we really gain anything by this. Although most #123 in text are probably not project references, the cost of doing a lookup for them is quite small, and some of them are.

In cases where users use #123 to refer to tasks in an external system, they can use a rule for that with higher precedence than this one or not give their projects conflicting hashtags.

Test Plan:

• This is well-covered by unit tests.
• Referenced a #3u1, per T9832.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9832

Differential Revision: https://secure.phabricator.com/D14547