Generate more friendly anchor names for header sections in Remarkup

Summary:
Depends on D20820. Ref T13410. We currently cut anchor names in the middle, don't support emoji in anchors, and generate relatively short anchors.

Generate slightly longer anchors, allow more unicode, and try not to cut things in the middle.

Test Plan: Created a document with a variety of different anchors and saw them generate more usable names.

Maniphest Tasks: T13410

Differential Revision: https://secure.phabricator.com/D20821