R1:9777c66576ff

Allow Phabricator to run with "enable_post_data_reading" disabled

Summary:
Ref T13008. Depends on D18701. The overall goal here is to make turning `enable_post_data_reading` off not break things, so we can run rate limiting checks before we read file uploads.

The biggest blocker for this is that turning it off stops `$_FILES` from coming into existence.

This //appears// to mostly work. Specifically:

- Skip the `max_post_size` check when POST is off, since it's meaningless.
- Don't read or scrub…