R1:1747b4d31854

Use "book" as the "Help" icon in the Remarkup toolbar

Summary: See D16811. I missed this while grepping because the other icon has two aliases (`life-buoy`, `life-ring`) and we were using one of each.

Test Plan: {F1912167}

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D16829