Bug 1052447 - [Messages] when we're in an activity and we delete all messages in the thread, we should close the activity r=azasypkin

Conflicts:
	apps/sms/js/thread_ui.js
Merge pull request #24337 from julienw/1052447-v2.0-fix-exit-activity-on-delete

Bug 1052447 - [Messages] when we're in an activity and we delete all mes...