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