rPHABRICATOREXTENSIONS277feb1b548d

Bug 1475626 Make the secure revision banner more clear about not modifying subscribers

Summary: Make the banner more clear that subscriber changes may be overwritten.

Test Plan: Observe banner change for private revisions.

Reviewers: mcote

Reviewed By: mcote

Bug #: 1475626

Differential Revision: https://phabricator.services.mozilla.com/D2578