R1:656a6f9b55ec

Fix editing of JSON config values via web UI

Summary: We should set the value if it's valid, not if it's invalid. derpp~

Test Plan: Set `files.viewable-mime-types`.

Reviewers: asherkin, codeblock, btrahan, vrana

Reviewed By: asherkin

CC: aran

Differential Revision: https://secure.phabricator.com/D4510