Merge pull request #18049 from etiennesegonzac/bug-993018-place-new-call-activity

Bug 993018 - Replace the "place new call" postMessages with a WebActivity r=rik
(cherry picked from commit d2a70ab329a0f85a1a7fd38c7c9297abd763ee1e)

Conflicts:
	apps/communications/dialer/js/dialer.js
	apps/communications/dialer/test/unit/call_screen_test.js
	apps/communications/dialer/test/unit/dialer_test.js