Added a setup check for empty REMOTE_ADDR

Summary: Fixes T8850. Previously, if a user's preamble script mangled $_SERVER['REMOTE_ADDR'] or somehow set it to null, the user would get errors when performing certain actions. Now those errors shouldn't occur, and instead the user will be warned that there is a setup issue related to their preamble script.

Test Plan: Create a preamble script that contains $_SERVER['REMOTE_ADDR'] = null; then navigate to /config/issue/. There should be a warning there about REMOTE_ADDR not being available.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, yelirekim, epriestley

Maniphest Tasks: T8850

Differential Revision: https://secure.phabricator.com/D16450