Move the Git LFS gate to dedicated (non-prototype) config

Summary: See PHI131. Ref T7789. Although this probably isn't 100% complete, there don't seem to be any actual, known, practical blocking issues remaining (everything is either heresay or not reproducible).

Test Plan: Tried to push LFS locally, got blocked with a helpful message. Enabled setting, tried to push LFS locally, got a successful push.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T7789

Differential Revision: https://secure.phabricator.com/D18825