Use "overflow-[x/y]: auto;" instead of "overflow-[x/y]: scroll" in Conpherence

Summary: `scroll` always shows the scroll bar, `auto` shows it only if scrolling needs to happen. Fixes some weirdness in the blank/empty states in Safari, at least.

Test Plan: Looked at Conpherence in Safari.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4635