When the autocompleter would fall off the bottom of the screen, put it above the text instead

Summary: Ref T10163. In cases like Conpherence, the autocompleter can currently render off the bottom of the screen. Put it above if it would be offscreen.

Test Plan: {F1062286}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10163

Differential Revision: https://secure.phabricator.com/D15038