R1:bff795d848b9

Allow disabling editing multiple files at once

Summary: Resolves T2095.

Test Plan:
Saved it, button disappeared.
Saved it back, button appeared.

Reviewers: epriestley, aran

Reviewed By: epriestley

CC: Korvin

Maniphest Tasks: T2095

Differential Revision: https://secure.phabricator.com/D4071