Shorten buttons on Leaving High Security Page

Summary: Changes the text to just "Stay", which is still obvious what it means, with less copy. Fixes T7027

Test Plan: Now works on mobile.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7027

Differential Revision: https://secure.phabricator.com/D12075