Show the default value in Config.

Summary:
As mentioned by @epriestley in an inline on D4290, we should show what happens
if the user leaves the box blank.

Test Plan: Went to edit a setting and saw the default below the text box.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, asherkin

Differential Revision: https://secure.phabricator.com/D4293