Refactor developer options to specific developer-mode option.

Summary: Refactor options related to verbose error reporting and forcing disk reads into a single developer option.

Test Plan: Run Phabricator with the developer-mode option set and check that errors print stack traces, static assets are always reloaded, etc.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4780