Add a configuration warning when `memory_limit` will limit file uploads

Summary: Fixes T6011. See that task for discussion. We can detect when `memory_limit` will be the limiting factor for drag-and-drop uploads and warn administrators about it.

Test Plan: Fiddled configuration values and hit, then resolved, the issue.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6011

Differential Revision: https://secure.phabricator.com/D10413