R1:74d6bcbdceb9

Allow a user to target "#anchor" by navigating to any prefix

Summary:
Ref T13410. We currently generate some less-than-ideal anchors in remarkup, but it's hard to change the algorithm without breaking stuff.

To mitigate this, allow `#xyz` to match any target on the page which begins with `xyz`. This means we can make anchors longer with no damage, and savvy users are free to shorten anchors to produce more presentation-friendly links.

Test Plan: Browsed to `#header-th`, was scrolled to `#header-three`…