Fix an issue in the new Harbormaster build log view where clicking the "^" icon doesn't work right

Summary:
Ref T13130. See PHI617.

The new build log UI has tags like <a href="...">Show More Above <span icon>^</span></a>. If you click the little "^" icon, the event target is the <span /> instead of the <a /> so we expand on the wrong node.

Instead, select the <a /> by sigil explicitly.

Test Plan: Viewed new log UI in Harbormaster, clicked "^" icon and text, got the same (correct) behavior on both.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13130

Differential Revision: https://secure.phabricator.com/D19410