Bug 883616 - Bug 882607 resolution introduced a XSS vulnerability r=gnarf

Carefully escape the body before injecting it into the document
Added some tests to ensure this for both SMS and MMS.
(cherry picked from commit 079b57f45f2e272ceb4e0f3f067cc3d829b3f2b4)

Conflicts:
	apps/sms/test/unit/mock_utils.js