R1:2f7010d18e4f

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…