R1:dc6b988deac9

Fix project hashtag regexp to stop matching terminal periods

Summary:
Fixes T6416. The comment is consistent with intent, but the actual regexp doesn't quite work right. In particular, we incorrectly match `#security.` as `security.` (with a period) instead of `security` (with no period).

Since this stuff is a pain to test and I evidently got it wrong in this case in D8703, make it unit testable.

Test Plan:
Added unit tests. Also:

{F227181}

Reviewers: chad, btrahan

Reviewed By: btrahan

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