Change "Stop" to "Pause" in Harbormaster build UI

Summary: Resolves T5814. Ref T1049. This changes "Stop" to "Pause" in the UI (internally it's still referred to as Stop).

Test Plan: Viewed builds and saw the intended wording.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T1049, T5814

Differential Revision: https://secure.phabricator.com/D10172