refactor activity-model.js: rename 'closed' to 'expired'
refactor activity-model.js: rename 'closed' to 'expired'

PowerlineApp/powerline-mobile,PowerlineApp/powerline-mobile,PowerlineApp/powerline-mobile,PowerlineApp/powerline-mobile,PowerlineApp/powerline-mobile,PowerlineApp/powerline-mobile,PowerlineApp/powerline-mobile,PowerlineApp/powerline-mobile