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:
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T10163
Differential Revision: https://secure.phabricator.com/D15038